A monoclonal antibody has been produced that recognizes the cytochrome P450 form, cytochrome P450IA1, but not cytochrome P450IA2 in rats and recognizes a single protein band of similar mol. wt on immunoblots of human liver microsomes. Immunohistochemical studies have been carried out with this antibody to investigate the localization and distribution of cytochrome(s) P450 of the P450IA family in human liver. Cytochrome P450IA was identified in both adult and fetal liver and in each case it was localized predominantly to hepatocytes. In adult liver there was a heterogeneous distribution of cytochrome P450IA immunoreactivity with cytochrome P450IA mainly present in zone 3 hepatocytes of the liver acinus. Within fetal liver there was a uniform distribution of cytochrome P450IA immunoreactivity with no apparent zonal distribution. Bile duct epithelium did not show definite immunostaining for cytochrome P450IA in either adult or fetal liver.