Thirty-seven cows affected with clinical mastitis (51 quarters) were injected iv with a single 0.02 mg/kg dose of Lydium-KLP. Fifteen cows were injected with both Lydium-KLP at the same dose given twice at a 24-hour interval and intramammary infusion of the agent (0.2 mg per quarter). It was found that the efficacy of the agent depended on the number of somatic cells in the mastitic milk and on their phagocytic activity at the beginning of the experiment.