Injection of highly purified staphylococcal alpha toxin into lactating rabbit mammary tissue produced dose-dependent areas of haemorrhagic necrosis and subsequent ischaemia. Injection of beta toxin produced transient dose-dependent erythema. High doses of alpha toxin destroyed the tissue architecture but infiltration with polymorphonuclear leukocytes did not occur. Injections of beta toxin produced intense erythema and infiltration with polymorphonuclear leukocytes. Mixtures of these toxins produced lesions which corresponded to the alpha toxin administered; the erythematous effect of beta toxin was inhibited. Comparison of these findings with those from sections obtained from experimental staphylococcal mastitis in rabbits suggested that the haemorrhagic necrotic syndrome known as "blue breast" was directly attributable to alpha toxin. The role of beta toxin, however, remains unclear.