Receptors specific for IgA were sought on mouse lymphoid cells by using a rosetting technique in which cells are exposed to modified ox red blood cells (ORBC) coated with IgA myeloma proteins having specificity for the modified ORBC. It was found that 4 to 6% of unseparated spleen lymphoid cells (pre-incubated at 37°C overnight) formed rosettes with MOPC 315-coated TNP-ORBC, with QUPC 52-coated dextran-ORBC, and with SAPC 10-coated arabinogalactan ORBC, whereas modified ORBC alone or modified ORBC mixed with irrelevant myelomas did not form appreciable numbers of rosettes; the cells forming rosettes were not neutrophils as shown by appropriate staining techniques. Thirteen to 22% of spleen cells also formed rosettes with IgG-coated ORBC (either IgG rabbit anti-TNP-coated TNP-ORBC or rabbit IgG anti-ORBC-coated ORBC); however, by using similar techniques, no cells forming rosettes with IgM-coated ORBC could be found. The specificity of the IgA-binding sites thus demonstrated was shown in inhibition studies: IgA antigen-antibody complexes formed near equivalence or, indeed, purified IgA were found to inhibit rosette formation with IgA-coated ORBC, whereas IgG complexes or purified IgG were found to cause no such inhibition. These inhibition studies were buttressed by depletion studies in which removal of cells binding IgG-coated ORBC with rosetting procedures was shown to have little or no effect on the percentage of cells binding IgA-coated ORBC in the residual cell population. The cells capable of binding IgA-coated ORBC are at least in part T cells in that the binding of IgA-coated ORBC is a property of nylon-column passed cells containing 2 fragments of IgA do not block IgA-rosette formation. In summary, we have identified an IgA-Fc-specific binding site on mouse lymphoid cells, including mouse T cells.