Mobilizable virulence plasmids of Yersinia enterocolitica of serotypes 0:3 and 0:9 were constructed by cointegration of a mobilizable vector into the virulence plasmids. The obtained cointegrates were mobilized into plasmidless Y. enterocolitica strains of serotypes 0:3, 0:5, 0:8, and 0:9. 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 0:3, 0:8, and 0:9), but they were not transferable to Y. enterocolitica antigen reference strains (serotypes 0:3 and 0:8) or to a plasmidless clinical isolate of serotype 0:5. A further striking result was that a serotype 0:8 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 0:3, 0:5, 0:8, and 0:9 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.