The products of I-region genes of the murine major histocompatibility complex (H-2) are intimately involved in the regulation of immune responses. In a number of antigen systems, suppressor T (Ts) cells and their factors (TsF) bear determinants of gene(s) that map to the I-J subregion of the H-2 complex. Suppression in one such system, poly(Glu50Tyr50)(GT), involves the interaction of multiple Ts subsets. Three monoclonal I-Jk-bearing GT-TsF have been identified that functionally differ from one another. We report the binding of these monoclonal GT-TsF to different anti-I-Jk monoclonal antibody columns. We find that these anti-I-Jk monoclonal antibodies display differing degrees of efficiency of GT-TsF binding. These data suggest a greater degree of heterogeneity of I-Jk gene products than has been proposed before.