Evidence for the existence of two molecular species of exfoliative toxin (ET) synthesized by Staphylococcus hyicus (SHET) under chromosomal and plasmid control is presented. Serological evidence that these molecular species of toxins are distinct from each other is given. The molecular weights of SHET from plasmidless strain P-1 (SHETA) and from plasmid-carrying strains P-10 and P-23 (SHETB) were almost equal. Both of the serotypes of SHET exhibited exfoliation in 1-day-old chickens. The plasmid-cured (P ؊ ) substrains (P-23C1 and P-23C2) of S. hyicus P-23 did not cause exfoliation in 1-day-old chickens, whereas P ؊ substrains (P-10C1 and P-10C2) of strain P-10 caused exfoliation, but they decreased their exfoliative activity. These findings suggest that SHETB was synthesized along with SHETA by strain P-10, whereas the P-23 strain synthesized SHETB alone. The plasmid-carrying strain (P-23) as well as the plasmidless strain (P-1) exhibited the typical clinical signs of exudative epidermitis in pigs. However, plasmid-cured (P ؊ ) substrains of P-23 (P23C1 and P23C2) did not exhibit the typical clinical signs of exudative epidermitis. These findings suggest that SHETA is synthesized under chromosomal control and SHETB is synthesized under plasmid control and that SHET-producing strains can be divided into three groups: SHETA-producing strains, SHETB-producing strains, and strains producing both toxins.Staphylococcus hyicus is known to be a causative agent of exudative epidermitis (EE) in pigs (28). EE is a generalized infection of the skin characterized by greasy exudation, exfoliation, and vesicle formation (7, 13).Amtsberg (1) has shown that the culture filtrate of S. hyicus contains an exotoxin that causes exfoliation in piglets. He has also suggested that the exfoliative activity of this exotoxin is similar to the exfoliative toxin (ET) produced by Staphylococcus aureus. We isolated this exotoxin from the culture supernatant of S. hyicus P-1 and designated it SHET (26). In 1993, we described some of the characteristics of purified SHET from the culture supernatant of S. hyicus (29). The molecular mass of SHET was estimated to be 27 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and it was determined that SHET differs from ETA and ETB in antigenicity and animal susceptibility (29).We have recently reported that the SHET-producing strain of S. hyicus causes EE in pigs while the non-SHET-producing strain does not and that SHET can be divided into more than two serotypes (30; H. Sato, T. Tanabe, T. Watanabe, K. Teruya, A. Ohtake, H. Saito, and N. Maehara, Proc. 14th IPVS Cong. Italy, p. 339, 1996). Andresen et al. (2, 3) have also reported that SHET has many serotypes. In this study, we detected the large plasmid in several strains of S. hyicus. We then confirmed the antigenic differences between SHETs from the plasmid-carrying strain and the plasmidless strain and between the SHET-producing ability of the plasmid-carrying strain and its plasmid-cured substrains. MATERI...