Three polypeptides (200, 46, and 25 kDal) encoded by the virulence plasmid were detected by SDS-PAGE in the outer membrane of Yersinia enterocolitica 09 grown at 37 degrees C in brain-heart infusion medium. Bacteria grown at the same temperature in the tissue culture medium RPMI 1640 expressed five additional polypeptides (170, 135, 118, 100, and 98 kDal), but the 25-kDal band was not seen. The protein profile in RPMI 1640 resembles the expression pattern displayed by yersiniae when grown in vivo. The immunoblot of total membrane proteins of bacteria grown in brain-heart infusion medium revealed eight plasmid-encoded polypeptides, four of which were also in the outer membrane preparations, including a 28-kDal polypeptide. These peptides do not coincide with known plasmid-encoded outer membrane proteins.