The effect of carminomycin on the activity of liver monohydroxygenases was estimated. It mas noted that in doses of 2, 3 and 4 mg/kg the antibiotic had no effect on the sleep duration in mice treated with hexenal. This meant that it did not change the activity of the enzymatic system. Induction and inhibition of liver monohydroxygenases did not influence the toxic effect of carminomycin used in doses of 5 and 10 mg/kg. It was also shown that the toxicity of both rubomycin and adriamycin decreased on induction and increased on inhibition of liver monohydroxygenases. Like adriamycin rubomycin lowered the activity of this enzymatic system. Administration of rubomycin in a dose of 5 mg/kg prolonged the duration of the hexenal sleep almost 1.5-2 times as compared to the control. The possible participation of liver monohydroxygenases in metabolic transformations of the antracycline antibiotics is discussed.