An infusion of E. coli endotoxin (10 μg) into the mammary gland of the cow 16 h before experimental infection with St. agalactiae prevented the establishment of mastitis. A clinical examination of the affected gland did not reveal signs of inflammation, all organisms were eliminated from the milk by 36 h after infection. Mammary glands not pretreated with endotoxin, and injected with the same number of viable streptococci, showed signs of severe inflammation, the milk yield was reduced and the injected pathogen could be isolated from the gland for up to 14 days when sampling was discontinued. The injection of endotoxin alone produced a rapid increase in the cell count of the milk, there were some signs of inflammation and 80% of the cells in the milk were neutrophils. The cell counts in the milk remained above normal for 7–10 days. Untreated control quarters appeared to be unaffected by the injection of E. coli endotoxin into an adjacent quarter, on the other hand the injection of St. agalactiae into the adjacent quarter on the same side of the udder produced a cell response which reached a peak after 2 days and returned to preinoculation levels by the fourth day. No inflammation was observed in these control quarters and no streptococci were isolated from the milk.