Mammary glands of mice were inoculated with a well-defined strain of Staphylococcus aureus or with derived mutants lacking alpha-haemolysin, coagulase or protein A, in order to evaluate the pathogenicity of these factors. Alpha-haemolysin-negative and coagulase-negative mutants showed less virulence than the wild-type strain. Various protein A-negative mutants gave contradictory results. The lesions ranged from consistently non-reactive necrosis of the entire mammary gland to a limited inflammatory reaction. The necroses, especially when affecting the whole mammary gland, were mostly associated with vascular lesions. Because the necroses were non-reactive, the vascular lesions were considered to be primary, but they could not be linked exclusively with the effect of alpha-haemolysin and a multifactorial aetiology seemed most probable. After inoculation with coagulase-lacking bacteria the development of parenchymal lesions was delayed, hypothetically because of increased phagocytic activity. The investigation indicated that protein A could have influenced the pathogenesis of the lesions, since all but one of the mutants lacking protein A showed low virulence, whereas a high protein A-producing, haemolysin-negative mutant was as virulent as the wild-type strain. A connection between protein A, bacterial adherence and bacterial growth rate is therefore conceivable. The possibility cannot be excluded, however, that during mutagenesis some as yet unknown cell surface factors were affected.