Virulent Yersinia species (Y. pestis, Y. pseudotuberculosis and Y. enterocolitica) possess a 70 kb virulence plasmid that encodes the Yop virulon. This virulence system allows extracellular bacteria adhering at the surface of eukaryotic cells to secrete and inject bacterial effector proteins, called Yops, into the cytosol of these cells in order to disarm them. These secreted Yop proteins are remarkably conserved among the different species. A Y. enterocolitica O:8 strain was found to secrete a protein antigenically related to YopM but significantly larger. Sequencing of the corresponding gene showed that the protein was a YopM variant with three repeats of one domain. Comparison of the yopM gene of various Yersinia strains by PCR amplification, as well as analysis of the secreted Yop proteins by SDS-PAGE and Western blotting revealed that, unlike the other Yops, the YopM protein shows some heterogeneity.