Fifty-eight Staphylococcus aureus strains isolated from bovine mastitic milk at 27 dairy farms in Japan during the period from November 1988 to May 1989 were examined for their productivity of virulence associated factors. The positive rates of the total isolates for various virulence factors were as follows: toxic shock syndrome toxin-1 (TSST-1, 27.6%), staphylococcal enterotoxins (SEs, 34.5%), alpha-haemolysin (74.1%), beta-haemolysin (65.5%), delta-haemolysin (12.1%), DNase (100%), egg-yolk factor (25.9%), clumping factor (70.7%) and protein A (58.6%). All of S. aureus isolates from peracute mastitis produced TSST-1, SEC, alpha-haemolysin and beta-haemolysin. While none of clumping factor and protein A were detected among peracute isolates, these factors were produced at a relatively high frequency by isolates from chronic mastitis. In coagulase typing, the most predominant type was VI (36.2%), and types IV, V and VIII were not observed. TSST-1 positive isolates showed interesting characteristics which all of the isolates produce both SEC and coagulase type VI but lack egg-yolk factor, clumping factor and protein A except for one isolate. We could infer that TSST-1 and SEC contribute to bovine mastitis, especially to peracute mastitis from the present study.