The production of toxic shock syndrome toxin (TSST) by Staphylococcus aureus isolated from mastitic cow's milk and farm bulk milk was examined by a reverse passive latex agglutination method (RPLA). TSST was detected in 25 (58.1%) of 43 isolates from clinical mastitic cow's milk, in 79 (76.7%) of 103 isolates from subclinical mastitic cow's milk, and in 95 (75.4%) of 126 isolates from farm bulk milk, respectively. When the quantity of TSST in the isolates was determined by RPLA, the titers ranged from 5 to 2560. TSST with RPLA titers of 640 to 2,560 was produced by 83 (30.5%) of 272 isolates tested. Almost all of the isolates showing RPLA titers of 640 and over produced enterotoxin C, whereas isolates showing titers of 5 to 320 produced enterotoxin C or both enterotoxin A and C. SDS-polyacrylamide gel electrophoresis and isoelectric focusing with immunoblotting showed that the TSST from bovine isolates had same molecular size (22 kDa) and isoelectric point (7.2) as TSST-1 from human isolates. These findings are consistent with previous reports.