Several types of IFNs were tested for their ability to suppress TCDD-inducible P-450 dependent mixed function oxidase (MFO) system in mouse primary hepatocytes. Mouse IFN-gamma (IFN-G) markedly suppressed EROD activity when added at the same time as TCDD (10 nM). The antagonism of EROD activity by IFN-G exhibited both a dose-(10-100 U/ml) and time-depedence. In contrast, mouse IFN-A/B was only moderately suppressive and only at high concentrations (500 U/ml). Rat IFN-G was even more selective than mouse, wherase human IFN-G had no activity.