Mobilizable virulence plasmids of Yersinia enterocolitica of serotypes O:3 and O:9 were constructed by cointegration of a mobilizable vector into the virulence plasmids. The obtained cointegrates were mobilized into plasmidless Y. enterocolitica strains of serotypes O:3, O:5, O:8, and O:9. The transfer experiments revealed the existence of two different subgroups of plasmid-associated traits. (i) Animal virulence functions (mouse lethality and conjuctivitis provocation) were only transferable to plasmid-cured derivatives of virulent parent strains (serotypes O:3, O:8, and O:9), but they were not transferable to Y. enterocolitica antigen reference strains (serotypes O:3 and O:8) or to a plasmidless clinical isolate of serotype O:5. A further striking result was that a serotype O:8 strain regained the mouse lethality trait after receipt of a plasmid from a strain not lethal to mice. These results demonstrate that plasmid-mediated animal virulence functions are not uniformly expressed within Y. enterocolitica. (ii) The second subgroup of plasmid-mediated traits (calcium dependency, surface agglutinogens, HEp-2 cell adherence, and protein release) were transferable to all Y. enterocolitica recipient strains tested (serotypes O:3, O:5, O:8, and O:9 of different origin). For the first time HEp-2 cell adherence and temperature-induced release of five major protein species are described as transferable traits.