The safety of and the immune response to simultaneous administration of live, enteric-coated adenovirus type 4 (ADV-4), type 7 (ADV-7), and type 21 (ADV-21) vaccines were studied. Volunteers (476 men), randomly assigned to four study groups, received three vaccines (ADV-4, ADV-7, and ADV-21), two vaccines (ADV4 and ADV-7), one vaccine (ADV-21), or no vaccine (placebo). Subjects were observed for three weeks, and no side effects due to vaccination occurred. The percentages of susceptible subjects (those entering the study with a neutralizing antibody titer of >1:2 to each vaccine virus received) who seroconverted to ADV-4 were similar in both groups that received ADV-4 vaccine (78% of 77 subjects and 74% of 76). However, in the group that received three vaccines, only 62% of 77 subjects seroconverted to ADV-7, compared with 79% of 76 in the group that received two vaccines (P < 0.05). Only 58% of 77 subjects in the three-vaccine group seroconverted to ADV-21, compared with 69% of 59 in the group that received one vaccine (P > 0.1).