A commercial vaccine for intramammary inoculation to prevent staphylococcal mastitis was tested under experimental conditions. Groups of cows which were vaccinated in all four quarters, in two quarters in the dry period and which were not vaccinated respectively were challenged intramammary with a mixture of alpha, beta, alpha + beta, haemolytic strains of Staphylococcus aureus during lactation. After challenge, severe mastitis developed in most cows. Unlike cows in the half- and non-vaccinated groups, the vaccinated cows did not show a rise in body temperature. The anti alpha-haemolysin titre in the serum was higher in the vaccinated than that in the non-vaccinated group but after challenge this difference was reversed. The half-vaccinated group showed titres which were between those of the other two groups. This experiment does not suggest that vaccination offers substantial advantages in acute staphylococcal mastitis. Besides having a beneficial effect on milk production, vaccination prevents fever, which is a useful phenomenon as regards the clinical picture as a whole.