Bovine trichomoniasis is a sexually transmitted disease caused by the flagellated protozoan Tritrichomonas foetus. A protective surface antigen was previously identified and immunoaffinity purified from T. foetus isolate D1 with cross-reactive monoclonal antibodies (MAbs) TF1.15 and TF1.17 (BonDurant, R. H., R. R. Corbeil, and L. B. Corbeil, Infect. Immun. 61:1385-1394, 1993). This antigen elicited antibody responses in the serum and cervicovaginal mucus of heifers. Thus, it may be useful as an immunodiagnostic reagent as well as a subunit vaccine. Conservation of the antigen in all strains would be crucial for either application. We investigated the conservation of this antigen among 36 isolates of T. foetus from Argentina, Costa Rica, and the United States using MAbs TF1.15 and TF1.17 in an enzyme-linked immunosorbent assay. MAb TF1.17 reacted with 32 of the 36 isolates, whereas MAb TF1.15 reacted with all of the isolates tested. One of the isolates which did not react with MAb TF1.17 (i.e., D1#3) was investigated further by Western blotting (immunoblotting) to determine the reason for the lack of reactivity with one of the two cross-reactive MAbs. The antigenic band that was reactive with MAb TF1.15 had a molecular mass slightly lower than that of the corresponding band from isolate D1, which reacted with both MAbs TF1.15 and TF1.17. Thus, at least a major portion of the antigen appeared to be conserved. This was confirmed in a study of heifers infected with isolate D1#3. The vaginal immunoglobulin A antibodies of these infected heifers reacted with the antigen of isolate D1 that was immunoaffinity purified with MAb TF1.17. Therefore, even though the epitope recognized by MAb TF1.17 was missing in the challenge isolate (D1#3), the heifers developed an immune response to the rest of the molecule. These results indicate that the major portion of the previously described protective antigen is conserved in different isolates of T. foetus. This portion contains the epitope that reacts with MAb TF1.15. Most isolates express the whole antigen, which possesses both TFl.15 and TF1.17 epitopes, but the few isolates that are missing the portion containing the TF1.17 epitope may still elicit an immune reponse to the conserved portion. Thus, the protective surface antigen is promising for use in immunodiagnosis or vaccination against bovine trichomoniasis.