The growth potential and the polypeptide composition of Yersinia enterocolitica serotype 0:3 isolated from patients with uncomplicated diarrhoea, reactive arthritis or septicemia were evaluated under different culture conditions. The expression of polypeptides varied with presence of the virulence-associated 40-48 Mdal plasmid, growth medium, growth temperature and gas composition of the culture (air, carbon dioxide, oxygen). Also the initial growth medium at 26 degrees C, before temperature shift to 37 degrees C, influenced the subsequent growth potential and expression of polypeptides. The plasmid encoded at least 7 polypeptides. This plasmid also inhibited the multiplication of bacteria under defined culture conditions. The dominating plasmid-encoded polypeptides were optimally expressed in air or oxygen-supplemented growth medium. The majority of the chromosomally encoded polypeptides were expressed independently of presence of the plasmid, whereas the expression of at least 8 were repressed by the plasmid. Five chromosomally encoded polypeptides were expressed only in carbon dioxide and five only in oxygen environment. These results indicate that Y. enterocolitica may express different molecules in different environments in vivo. This may be of importance for host-parasite relationship and immune response.