Two Fc receptor-dependent tests were investigated to study the question of a relationship between Fc receptors and known cell surface antigens, in particular I region-associated (Ia) antigens: (a) a rosette assay with antibody-coated erythrocytes (EA) as indicator cells and normal mouse lymphoid cells as source of rosette-forming cells, and (b) a cytotoxicity test with antibody-coated erythrocytes as target cells and normal mouse spleen cells as a source of cytotoxic cells (K cells). EA rosettes were specifically inhibited by antibodies reacting with Ia antigens. Various other antisera reacting with antigens on B lymphocytes, like anti-Ly 4.2 (raised in H-2 identical mice), rabbit antimouse B-cell serum, or rabbit antimouse immunoglobulin, also specifically inhibited the rosettes. No inhibition occurred in the presence of allogeneic or xenogeneic antisera reacting with T lymphocytes. K-cell cytotoxicity was specifically inhibited by each of the antisera (reacting with either B cells or T cells). F(ab')2 fragments of anti-Ia antibodies could still specifically inhibit EA rosettes but they could not inhibit K-cell cytotoxicity. Similar results were obtained with F(ab')2 fragments of anti-immunoglobulin antibodies. These results indicate that the mechanism of inhibition of Fc receptors in the two tests was different. In neither of the tests could we find any evidence for a unique association between the Fc receptors and Ia antigens. The Fc receptors on K cells did not seem to be associated at all with Ia antigens.