Antibodies against cytochrome P-450 are found in some children with autoimmune hepatitis (antilivermdney microsome,) and in patients with ticrynafen hepatitis (antiliver/kidney microsome,). For an immune reaction against cytochrome P-450 to possibly destroy the hepatocytes, one must assume that cytochrome P-450 is present on the plasma membrane surface of hepatocytes. In a first series of experiments, plasma membranes were prepared with a technique based on the electrostatic attachment of isolated hepatocytes to polyethyleneimine-coated beads. After vortexing, beads were coated with a very pure plasma membrane fraction. Microsomal contamination, judged from the specific activities of glucose-6phosphatase or NADH-cytochrome c reductase, was less than 1%. Nevertheless, the specific content (per milligram of protein) of CO-binding cytochrome P-460 was 20% of that in microsomes; the specific be--(a)pyrene hydroxylase activity was 25%, and ethoxycoumarin deethylase 11%. Immunoblots showed the presence of cytochromes P-450 UT-A, UT-H, PB-B, ISF-G and PCN-E, the last three isoenzymes being inducible by, respectively, phenobarbital, 3-methylcholanthrene and dexamethasone. In a second series of experiments, nonpermeabilized isolated hepatocytes from untreated rats were incubated with anticytochrome P-460 antibodies. Immunofluorescence and immunoperoxidase staining confirmed the presence of cytochromes P-460 UT-A, PB-B and ISF-G on the membrane. In a last series of experiments, human antiliverkidney microsomal, antibodies were found to react specifically with rat liver plasma membrane cy- tochrome P-450 UT-H (IID subfamily). We conclude that several cytochrome P-450 isoenzymes are present, active and inducible on the plasma membrane surface of hepatocytes. It is therefore conceivable that immunization against plasma membrane cytochrome P-450 might lead to the immunological destruction of hepatocytes in some patients. (HEPATOLOGY 1990; 11:850-868.)