The Mls locus on chromosome 1 controls the expression of cellular determinants that are responsible for stimulating mixed lymphocyte reactions between H-2-identical strains. However, the biochemical nature of Mls antigenic determinants remains undefined. It has been proposed that Ly-17 lymphoid cell surface antigens (also known as Ly.m.20.2 and LyM-1) and Mls antigens could be identical because they are both encoded by loci on chromosome 1 and display similar tissue distribution. The Ly-17 locus encodes polymorphic alleles of the IgG Fc receptor (Fc gamma R). In the present study, two approaches were used to address the question of whether Fc gamma R are involved in T cell recognition of Mls antigen. In the first approach we tested the effect of Fc gamma R blockade by heat-aggregated mouse IgG or anti-Fc gamma R monoclonal antibodies (2.4.G2) on the ability of an Ia+, Fc gamma R+ Mlsa-expressing B cell hybrid (LBB.3.4.16) to stimulate interleukin 2 secretion by an anti-Mlsa-specific T cell hybrid. We show that blockade of Fc gamma R does not inhibit the Mlsa-specific stimulation of T cells during a 24-h culture period in which Fc gamma R remain blocked. In the second approach, we derived irradiation-induced variants of LBB.3.4.16 to dissociate Fc gamma R expression and Mls antigen expression. We describe 2 LBB variants which no longer stimulate Mlsa-reactive T cells but do express Fc gamma R. Compared to parental LBB cells, the capacity of variant LBB cells to present soluble antigen to Ia-restricted T cells is unaffected. Collectively, these results indicate that Fc gamma R expression and Mlsa antigen stimulation can be dissociated. We conclude that Fc gamma R expression may be necessary, but not sufficient for T cell recognition of Mls antigen.