For testing biological response of newborn Taiwan monkeys to infection of human adenovirus type 12 (Ad12) and simian virus 40 (SV40), 40 newborn Taiwan monkeys were inoculated with Ad12, Ad12, plus SV40, Ad12 supplemented with Ad12-induced hamster tumor tissue (Ad12 tumor) or control specimens (HeLa or African green monkey kidney cell lysate). Among them 26 survived including 8 newborn monkeys inoculated with control specimens. The survivors were observed for 4 years but no tumor was produced. The increase of body weight and intake of calories and protein in each test group during the first 12 wk were similar to those of corresponding control groups. Intrapulmonary inoculation of 108.2TCD50 of Ad12 with additional subcutaneous dose of Ad12 (108.8TCD50), Ad12 plus SV40 (108TCD50) or Ad12 plus Ad12 tumor killed 78% of newborn monkeys (7 of 9) in 18 days. The newborn could stand subcutaneous inoculation of SV40 (108TCD50) with 1 dose of Ad12 (108.8TCD50) or 3 doses of Ad12 (108.5, 108.5 and 108.2TCD50) at 24-hr intervals. When 108.8TCD50 or more Ad12 were inoculated, the virus could be isolated as late as 44 and 26 days from rectal and throat swab specimens respectively. The Ad12 neutralizing antibody in baby monkeys inoculated with multiple doses of Ad12 persisted, in low titer, longer than those injected with single high doses of Ad12, but anti-Ad12 T (tumor) antibody disappeared by 35 wk in both groups. Although SV40 antibody response was better than Ad12 antibody response in baby Taiwan monkeys, pre-infection of SV40 did not potentiate the production of anti-Ad12 T antibody.