The combined effect, if any, of salinomycin poisoning and salinomycin-tiamulin interaction on lipid-peroxidative processes and the antioxidative defence system of the liver was studied in domestic fowl. Male broilers (28-day-old), reared on a diet containing 60 mg/kg salinomycin, were treated intraoesophageally with salinomycin (140 mg/kg body mass) or tiamulin (50 mg/kg body mass). Malondialdehyde, reduced glutathione and cytochrome P-450 concentrations as well as glutathione peroxidase and catalase activities of the liver were determined. Liver malondialdehyde concentration rose in the salinomycin-treated group while the amount of cytochrome P-450 increased in both groups treated. Glutathione concentration and glutathione peroxidase activity of the liver decreased rapidly but hepatic catalase activity increased in both groups after the treatment. Manifestation of the effect exerted by salinomycin and salinomycin-tiamulin on lipid-peroxidative processes nearly coincided with the onset of clinical signs and preceded the increase of hepatic cytochrome P-450 concentration. According to the results, the background of the previously reported incompatibility between salinomycin and tiamulin is the synergistic effect exerted on the antioxidant (glutathione) system.