Martin Arrowsmith is the subject of the novel. His development from young, curious boy to serious scientist is the subject of the story. Martin begins his journey as the unpaid assistant to drunken Doc Vickerson in midwestern America. He goes off to medical school. There the idea of working in a laboratory like his hero and mentor, Professor Max Gottlieb, takes hold. He starts down the path of becoming a scientist, but life intervenes. He falls in love and gets married. With a wife to support, he takes a job as a country doctor, and then works in public health, before finally getting back on track to becoming a scientist. Throughout Martin's life, with its successes and failures, he is pulled away from his goal by other forces. The temptation to make money, be respected and admired, and socialize with the wealthy is always present. The frustration of other people's judgment and criticism is similarly distracting. And he must grapple with two competing vocations—being a physician and being a scientist. These are not as compatible as one would think. A physician heals patients. A scientist conducts experiments with rigid controls to discover answers. Though these two vocations often intersect, they can be at odds. Martin faces up to this conflict on the island of St. Hubert, where the plague is spreading. He is supposed to conduct an experiment by treating half the people with his phage and allowing the untreated half to be the control group. But this means that if the phage is effective, people will die because he chose to not treat them. People do die, and Martin finally gives in and treats all those he can. Martin feels that, though he saved lives, ultimately his trip to St. Hubert was a failure. He can't be both physician and scientist. Once he sees this clearly and chooses to be a scientist, his real work begins.
When Martin first meets Leora Tozer, she is a student nurse at the hospital near the University of Winnemac. Though he is engaged to the refined Madeleine Fox at the time, he quickly falls for the more unpolished Leora. He feels she is his kind of people, like they are two halves of a whole. He soon becomes engaged to her as well and must break things off with Madeleine. After Leora becomes Martin's wife, she plays a vital role in his life and development. She unconditionally loves him, though she is aware of his faults. She patiently follows him through the twists and turns of life and career and rarely complains. She makes the best of each new situation. She also keeps him from straying too far from his true self. When he gets tangled up in trying to be respectable, she reminds him he is a barbarian at heart. Leora accompanies Martin to St. Hubert where he is to study the effectiveness of his phage on the plague. There she contracts the disease and dies.
Max Gottlieb is a bacteriologist, and Martin Arrowsmith's most important mentor throughout the book. He is a German Jew who has come to the United States. He has a wife and three children. When Martin first encounters Gottlieb, the scientist is a professor of bacteriology at the University of Winnemac. His reputation for long nights in the lab and antisocial behavior intrigues Martin, who sees him as a hero. Gottlieb teaches Martin his first lessons in what real science is and the dedication and temperament needed to pursue it. The novel develops Gottlieb's story alongside Martin's even after Martin leaves the university and begins his career. As Martin becomes a country doctor and then a public health official, Gottlieb goes to work in the pharmaceutical industry. However, Gottlieb's belief that medicines should not be patented for profit puts him at odds with the lucrative industry. He ends up at the McGurk Institute, and when Martin is in need of a job, he invites Martin to join him there. When Martin goes off to St. Hubert, Gottlieb reminds him to stay true to the controlled experiment they planned, in which half of the population would not get the plague medicine and half would. Gottlieb stays true to science as his religion throughout the book, providing an example of what Martin hopes to be and struggles to become.
Terry Wickett enters the novel when Martin Arrowsmith goes to work at the McGurk Institute in New York. Wickett is a chemist working with Gottlieb. Martin immediately takes a dislike to him, thinking him rude and his voice grating. But Wickett soon shows himself to be a dedicated and clever scientist. He is unmarried and devotes all his time to his work, often stays up all night in the lab. Martin realizes his rough, rude ways are simply brutal honesty mixed with Wickett's own sardonic sense of humor. Wickett and Martin begin to become friends, staying up late together in the lab and sharing ideas. The friendship between the two men is not all smooth sailing. Wickett's dream is to build a lab in the woods and work there unhindered by the concerns of the world. When Martin is torn between joining him and staying with his wife, Wickett becomes surly. But eventually Martin chooses working with Wickett over his marriage to Joyce, and the two scientists set forth on a new phase of life.
Gustaf Sondelius enters Martin Arrowsmith's life when Martin goes to a lecture by the famous "soldier of science." Sondelius is a wealthy man, but he chooses to spend his time traveling the world "founding institutions and making inconvenient speeches and trying new drinks." He is extroverted and enthusiastic. Sondelius inspires Martin to dream of doing big things, like curing terrible diseases. He remains an influence on Martin as Martin begins his career as a doctor. When Martin is at the McGurk Institute working on the phage, Sondelius arrives and becomes his assistant. This might seem like a humble post for a world-famous epidemic fighter. But Sondelius is interested in the work, not in boosting his ego. When Martin goes to St. Hubert, Sondelius goes along. He and Martin have a disagreement over whether to give all or only half the people the plague-fighting phage. Sondelius says he will not take the phage until Martin gives the medicine to all the people of St. Hubert. Despite this disagreement Sondelius throws himself heroically into the role of rodent killer, killing rats and ground squirrels that transmit plague. He contracts the disease as a result and dies on St. Hubert.