1. In previous studies (Boobis et al., 1985b) it was shown that a monoclonal antibody (MAb 3/4/2), raised against rat cytochrome P450 form c, reacts with an isoenzyme(s) of cytochrome P450 in human liver. It was predicted that the epitope with which this antibody reacts should be present on both isoenzymes of the P450IA gene sub-family (the orthologues of forms c and d) in man (Edwards et al., 1987). 2. This antibody was used to probe 45 different samples of human liver, by the technique of Western blotting. With one exception, all of the samples contained immunoreactive protein, a single band at Mr 54,000 (orthologous to rat form d), which ranged in content from less than 0.5 to 33.5 pmol mg-1 microsomal protein. The content of the human orthologue of form c was below 0.5 pmol mg-1, the limit of detection of the assay. 3. Thirteen of the samples were from patients of known smoking status. Immunoreactive P450 content was 3.5-fold higher, and phenacetin O-deethylase activity was four-fold higher, in the smokers than in the non-smokers. 4. There was a highly significant correlation between the amount of immunoreactive cytochrome P450 and the high affinity component of phenacetin O-deethylase activity in both smokers and non-smokers. 5. It is concluded that the high affinity component of phenacetin O-deethylase activity in man is catalysed by the orthologue of rat cytochrome P450d, and that this isoenzyme is inducible by cigarette smoking. 6. In a number of previous publications it has been suggested that there is an association between the poor metaboliser (PM) phenotype for debrisoquine and impaired phenacetin O-deethylation. In the present study it was shown that not all subjects PM for debrisoquine are poor metabolisers of phenacetin.