A simple technique was developed for the intramammary inoculation of mice. Experimental mastitis was produced in lactating mice with strains of Staphylococcus aureus, Streptococcus agalactiae, Corynebacterium pyogenes, Pseudomonas aeruginosa and Escherichia coli that had originally been isolated from cases of bovine mastitis. There was evidence that the spectrum of pathogenicity of the different bacterial strains in the bovine species was to some extent paralleled by differences in pathogenicity in the mouse. Mastitis could be produced in BSVS mice by the inoculation of comparatively small numbers of staphylococci. The appearance and histology of the affected mammary glands resembled mastitis produced in the bovine species by the bacterial species concerned. The size of the mouse mammary gland was very convenient for histological and other studies requiring examination of the whole gland. Experimental mastitis in the mouse provides a useful model for certain fundamental studies and for experiments, such as those of a screening nature, prior to studies on cattle.