The effects of chloramphenicol treatment to rats, by intraperitoneal way on three consecutive days were studied on various enzymatic systems of hepatic microsomes, implicated in the detoxication reactions non induced or induced by phenobarbital. Chloramphenicol increases the activity of reductases like NADPH cytochrome c reductase and neotetrazolium reductases. It does not appear to modify the activity of the N-demethylase systems or of the cytochrome P 450 level contrarily to phenobarbital. Chloramphenicol decreases the activity of microsomal nitro reductases, the production of malonaldehyde and conjugated dienes in the microsomes currently considered as a reliable estimate of the extent of the lipoperoxidation process. These results suggest that chloramphenicol could act like an antioxdant derivative.