After completing her undergraduate degree, Steitz applied to medical school rather than graduate school since she knew of female medical doctors but not women scientists. She was accepted to Harvard Medical School, but having been excited by a summer working as a bench scientist in the laboratory of Joseph Gall at the University of Minnesota, she declined the invitation to Harvard Medical School and instead applied to Harvard's new program in biochemistry and molecular biology. There, she was the first female graduate student to join the laboratory of Nobel Laureate James Watson, with whom she first worked on bacteriophage RNA.