Cross-immunization experiments have revealed a significant antigenic diversity of the isolate of Cowdria ruminantium which needs to be characterized for the development of vaccines. We identified polymorphic immunodominant antigens by ELISA and immunoblot. Using serum from a goat immune to the Gardel stock of Cowdria (isolated in Guadeloupe) adsorbed on antigen of the Senegal stock of this pathogen, distinct serogroups were revealed by ELISA among six isolates from different geographical origins. Furthermore, a goat serum directed against the Senegal stock and adsorbed on Gardel antigens was shown to be specific for the Senegal stock, thus confirming the existence of serotypes in Cowdria. The Major Antigenic Protein 1 (MAP1) of Cowdria was shown to have variable antigenic determinants. Also in a group of variable proteins ranging from 23 to 29kDa, one antigen of 26-27kDa had a determinant specific for the Gardel isolate. These polymorphic antigens may be relevant components of Cowdria ruminantium for a vaccine as the sera revealing these antigens originated from a goat surviving a lethal challenge. However, the presence of T-cell epitopes and the ability of these antigens to confer protection to ruminants remain to be investigated. The production of a rabbit antiserum against this group of polypeptides will be of great use for their purification and for the screening of expression libraries.