Despite the widespread inclusion of diversity-related curricula in US medical training, racial disparities in the quality of care and physician bias in medical treatment persist. The present study examined the effects of both formal and informal experiences on non-African American medical students' (N=2922) attitudes toward African Americans in a longitudinal study of 49 randomly selected US medical schools. We assessed the effects experiences related to medical training, accounting for prior experiences and attitudes. Contact with African Americans predicted positive attitudes toward African Americans relative to White people, even beyond the effects of prior attitudes. Furthermore, students who reported witnessing instructors making negative racial comments or jokes were significantly more willing to express racial bias themselves, even after accounting for the effects of contact. Examining the effects of informal experiences on racial attitudes may help develop a more effective medical training environment and reduce racial disparities in healthcare.