(1) Ten monoclonal antibodies have been raised against human hepatic microsomal proteins, seven of which recognised a purified human liver cytochrome P450, P450hA7. (2) Two of the seven anti-P450hA7 monoclonal antibodies, when blotted against control and induced rat liver microsomes, recognised a male-specific, weakly expressed constitutive protein that showed marked induction by pregnenolone-16-carbonitrile (PCN) but not by phenobarbitone (PB). No such P450 has been previously reported. (3) Another of the anti-P450hA7 monoclonal antibodies recognised a protein that was not expressed in control, male or female, rat liver microsomes but was strongly expressed in both PCN- and PB-induced microsomes. This protein may correspond to cytochrome P450PCN1 which exhibits identical induction characteristics. (4) A third rat protein, strongly and constitutively expressed in male liver microsomes, is recognised by other anti-P450hA7 antibodies. This protein may be induced by PCN. (5) The hepatic microsomal level of immunoreactive P450hA7 in fifteen adult individuals showed marked interindividual variation and was approximately ten times higher in an epiletic chronically treated with the drugs PB, phenytoin, carbamazipine and valproate. (6) Foetal human liver microsomes contained a protein that was immunochemically similar, but not identical, to adult P450hA7. The foetal protein exhibited a slightly greater molecular mass than the adult form. The switch from the foetal to the adult form of P450hA7 occurred shortly after birth. (7) P450hA7 was immunochemically detected in HEP G2 human hepatoma cells. The cytochrome was constitutively expressed being present in cells treated with PB, PCN and benzanthracene as well as untreated cells. (8) The anti-P450hA7 antibodies have been put to a variety of applications including the immunohistochemical localisation of cytochrome P450hA7 in human tissue and the screening of human hepatic cDNA libraries in gtll. (9) These monoclonal antibodies constitute a precise and powerful tool for the further characterisation of the human cytochromes P450.