Polyethylene (PE) is degraded in microsomal fractions of mouse and rat livers with the formation of carbonyl groups. Infrared spectroscopy demonstrated formation of predominantly ketone groups and to a lesser degree ester and aldehyde groups. The inhibition and stimulation of cytochrome P-450 in mouse livers affected the formation of oxidative groups on PE. Phenobarbital doses of 3 x 0.05 mg per mouse increased the concentration of cytochrome P-450 and ketone groups on PE, whereas the vaccine Propionibacterium acnes (0.5 mg) and its pyridine fraction (0.5 and 1 mg) had the opposite effect. The coherence of cytochrome P-450 with oxidative changes on PE is compared and discussed with findings on implants in man.