Three strains of bacteria which induce reactive arthritis were collected: a Shigella flexneri, designated 7060; another Sh. flexneri, designated 316; and a Yersinia enterocolitica of serotype 03. Rabbit antisera were generated against each of them to test for the extent and nature of cross-reactivity among these strains. When analysed by the ELISA technique, antisera against 7060 and 316 showed strong cross-reactivity with Y. enterocolitica. In contrast, the reaction of antisera prepared against putatively non-arthritis-causing bacteria reacted several-folds less. Using immunoprecipitation, sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western Blot procedures, a 92,000 MW cross-reactive antigen on the Yersinia was identified. The antigen was present on the outer membranes of the Y. enterocolitica, and enzyme digestion experiments showed that this antigen was protein in nature.