AbstractStreptolysin O, a thiol-activated exotoxin from group Aβ-haemolytic streptococci, caused a dose-dependent depression of aniline hydroxylase, aminopyrine N-demethylase and ethylmorphine N-demethylase activities when added into the hepatic microsomal mixtures from male rats at concentrations 0.02 -0.4 HU/mLin vitro. the activities of 7-ethoxycoumarin O-deethylase, 7-ethylresorufin O-deethylase and 7-pentylresorufin O-depentylase were not altered with the used concentrations of the toxin. Specific antibody against haemolytic action of streptolysin O added to incubation mixturesin vitrowas not able to protect streptolysin-sensitive monooxygenases from the inhibition. the addition of streptolysin 0 (0.01 -0.8 HU/mL) into the cytosol-containing medium did not significantly influence the activity of procainamide N-acetyltransferase. Immunomodulators picibanil (OK 432) and human recombinant interferonα2A which are known to suppress oxidative metabolismin vivoin humans and animals, were without effect either on the cytochrome P-450-dependent monooxygenases or on the N-acetyltransferase activity when administeredin vitroat the doses real in their clinical application (0.001 -0.1 KE/mL of picibanil and 10 -500 U/mL ofα-interferon).