A polymorphic determinant on the surface of canine peripheral blood mononuclear cells (PBMC) has been identified using the murine monoclonal antibody W3G10. The determinant, which has a molecular weight of approximately 39 Kd, is associated with DLA-Dw1 (w = workshop designation), -Dw3, -Dw4, -Dw10, -Dsea1 (sea = Seattle designation) and -Dsea9, but not -Dw8, -Dw9, -Dsea4, -Dsea10, -Dsea16 or -Dsea17, and segregates in Mendelian fashion. There does not appear to be any linkage disequilibrium with currently recognized DLA-A, -B or -C alleles. Cytotoxic effector function was eliminated when W3G10 positive alloantigen primed PBMC were treated with W3G10. However, cytotoxicity was not affected when W3G10 negative PBMC were used, thus further confirming polymorphism. The data suggest that the antigen identified by McAb W3G10 is not a typical class I or class II MHC antigen, but may be encoded by a closely linked gene.