The level of cytochrome P-450 and the oxidation of aminopyrine and benzo(a)pyrene hydroxylase were depressed in hepatic microsomes prepared from mice infected with the gram positive bacteria Listeria monocytogenes. Maximum depression of mixed function oxidase occurred on the 2nd day of infection. This loss in drug biotransformation capacity in the liver was correlated directly with the number of organisms found in that organ. The ability of mice to metabolize drugs in vivo also was impaired during Listeria monocytogenes infection. During the infective period the half-life of theophylline was significantly prolonged and the N-demethylation of aminopyrine as measured by the expiration of 14CO2 from radiolabeled aminopyrine was diminished. The loss of drug metabolism was not due to interferon production, fever or morphological damage to the liver. These results indicate that certain bacterial infections can depress drug biotransformation and elimination in a similar manner to that already reported in viral and parasitic infections. This finding may be of significance to patients receiving drugs which are metabolized by the mixed function oxidase system during episodes of infection with some bacteria.