1. Aflatoxin B1 (1.5 mg/kg body weight, i.p.) was administered to rats, mice, quail and chickens to examine the comparative effect on hepatic microsomal drug-metabolizing enzymes, cytosolic glutathione S-transferase and serum enzymes. 2. Administration of aflatoxin B1 to rats resulted in a significant decrease in microsomal cytochrome P-450, NADPH-cytochrome c reductase, activities of aminopyrine N-demethylase, aniline hydroxylase, cytosolic glutathione S-transferase and liver glutathione content. However, no significant changes in these parameters were seen in mice. 3. Quail showed a significant decrease in the content of cytochrome P-450 and the activities of aminopyrine N-demethylase, aniline hydroxylase and cytosolic glutathione S-transferase. A similar treatment did not affect these biotransformation enzymes in chickens. 4. The activities of serum enzymes, sorbitol dehydrogenase, alanine aminotransferase and aspartate aminotransferase were increased significantly in rats and quail. Mice exhibited a significant increase in the activities of sorbitol dehydrogenase and aspartate aminotransferase, while chickens showed a significant increase only in alanine aminotransferase.