The interaction of the organotin fungicide triphenyltin chloride (TPT) with fish microsomal monooxygenase systems has been studied in vitro and in vivo in the marine fish scup (Stenotomus chrysops). In vitro incubation of fish liver microsomes with TPT resulted in the conversion of about 40% of the native total spectral P450 to P420. In addition, a strong concentration-related inhibition of ethoxyresorufin O-deethylase (EROD) activity was observed, with a complete loss at 1.0 mM TPT. Pentoxyresorufin-O-dealkylase (PROD) activity was inhibited only at the highest concentration tested. This suggests either some specificity for the EROD catalyst CYP1A1, or a loss of reductant NADPH cytochrome c reductase as the cause. Further in vitro incubations showed that NADPH, but not NADH, cytochrome c reductase was strongly inhibited at 100 microM TPT and higher. To further investigate this effect, fish were injected with single doses of 5, 25 and 50 microM TPT (1.9, 9.6 and 19.3 mg kg-1 TPT), and 24 and 48 h later, hepatic microsomes were analyzed for total P450 content, EROD activity, NAD(P)H cytochrome c reductase, and the content of three CYP forms. EROD activity tended to be decreased in TPT-treated scup, with the response being stronger after 48 than 24 h. No significant conversion of spectrally determined P450 to cytochrome P420 was found, and cytochrome b5 was not affected. However, both NAD(P)H cytochrome c reductases were significantly inhibited at all concentrations. Immunoblot analysis showed reduction of CYP1A1 content at all doses, being significant at 25 mM after 48 h, but no decrease in CYP3A-like protein, the dominant catalyst of testosterone 6 beta-hydroxylation, nor CYP2B-like protein, the major contributor to indicates significant effects of TPT at high concentrations on fish hepatic CYP1A1 protein, EROD activity and the reductases. TPT seems to act more specifically on CYP1A1 than on other CYP forms. These findings combined with those of our previous studies (Brüschweiler BJ, Würgler FE, Fent K. Environ Toxicol Chem 1996;15:827-735; Fent K, Bucheli TD. Aquat Toxicol 1994;28:107-126; Fent K, Stegeman JJ. Aquat Toxicol 1991;20:159-168; Fent K, Stegeman JJ. Aquat Toxicol 1993;24:219-240) indicate a general degenerative effect of organotins on the fish microsomal monooxygenase system, although some differences are seen between the organotins, and between species. We conclude that these effects of organotins have consequences for use of CYP1A as a biomarker and endocrine disruption.