Adult female B6C3F1 mice were gavaged with 4,4'-thiobis-(6-t-butyl-m-cresol) (TBBC) in corn oil at doses of 10, 100, or 200 mg/kg daily for 14 consecutive days. There was no overt toxicity, as manifested by grossly observable behavioral changes, decreased growth rate over the exposure period, or mortality. There were also no marked effects on serum chemistries or hematology, with the exception of a significant increase (41%) in the number of leukocytes at the highest dose. Absolute differential counts indicated that significant increases occurred in the number of lymphocytes (31%) and neutrophils (177%). Studies with bone marrow indicated a significant 30% increase in the number of cells/femur from animals treated with the highest dose of TBBC. The number of macrophage progenitors (CFU-M)/femur was significantly increased by 28%, while the number of granulocyte-monocyte progenitors (CFU-GM)/femur was nonsignificantly increased by 20% in the high dose animals. The weight of both the spleen and liver was increased in a dose-related fashion, although the histopathology of the spleen of TBBC-treated mice was not different from control. The livers of mice receiving the high dose showed mild focal hydropic degeneration, mild hepatitis, and a slight increase in the number of Kupffer cells. No other organs were affected. Liver microsomal protein and cytochrome P-450 levels were increased in a dose-related fashion. Enzyme activities of aminopyrine demethylase and aniline hydroxylase, but not arylhydrocarbon hydroxylase, were also increased in a dose-related fashion.