SUMMARY—Groups oF approximately 25 male and 25 female Swiss mice received injections oF 200 pg 4-aminobiphenyl, 4-amino-3-hydroxybiphenyl, 4-hydroxylaminobiphenyl, or 4-amino-4'-hydroxybiphenyl on each of the first 3 days of life. The materials were injected as solutions/ suspensions in 3% aqueous gelatin. A large control group was treated with the vehicle only and a further control group left untreated. As a positive control, 20 pag of 7,12-dimethylbenzIakinthracene (DMBA) was injected into 49 newborn mice on each of the first 3 days of life. In males, a marked and significant increase in the incidence of hepatomas above the control level was seen in response to 4-aminobiphenyl itself and to each of its three derivatives. In females, a slight but probably significant increase was noted in response to three of the test compounds, but not to 4-amino-3-hydroxybiphenyl. In neither sex was there an increase in the incidence of neoplasms at other sites. DMBA-treated mice of both sexes developed, as expected, pulmonary tumors (59%) and lymphomas (1 4%). In addition, a high incidence of hepatomas was recorded in the males, but none in the females. The results with regard to the four test substances are of interest because tests for carcinogenicity in other systems have given negative results and because the liver is the sole target organ. Further investigation of the difference in response of the two sexes to neonatally injected carcinogens is overdue. The results suggest that no evaluation of carcinogenicity may be complete unless it includes tests in neonates.—J Nat Cancer Inst 41: 403-410, 1968.