SUMMARYThe effects of culture supernatants conditioned by the growth of Staphylococcus aureus M60 on in vitro growth and functional properties of bovine mammary myoepithelial cells were examined. Myoepithelial cell proliferation was reduced by Staph, aurexis M60 culture supernatants. Exposure of myoepithelial cells to culture supernatants of isogeneic mutants of Staph, aureus M60 that produced either α or β toxins reduced proliferation, but to a lesser extent than supernatants from the wild type strain. Thus, α and β toxins may play some role in affecting myoepithelial cell proliferation. Of the cells tested, 42% contracted following addition of oxytocin (10^–7 M) in the culture medium. Treatment of myoepithelial cells for 15 min with Staph, aureus M60 supernatants, prior to addition of oxytocin in the culture medium, increased the number of cells that contracted to 92%. Exposure of cells for 3 h to the same supernatant, prior to addition of oxytocin in the culture medium, abolished oxytocin responsiveness, had no effect on immunolocalization of actin and vimentin, but affected the localization of α-actinin within myoepithelial cells. Treatment of myoepithelial cells for 3 h with a combination of purified staphylococcal proteinases XVI and XVII-B abolished oxytocin responsiveness and mimicked the effect of the Staph, aureus culture supernatant. We conclude that Staph, aureus M60 culture supernatant affected proliferation and functional properties of myoepithelial cells.