Several methods have been used in the present study to characterize Fc receptors (FcR) expressed on T-T hybridomas derived from mouse Peyer's patch T helper (Th) cell clones that preferentially support IgA responses. These T hybridomas (designated Th HA cells) produce IgA-binding factor (IBFα) which regulates antigen-dependent IgA responses. The ultrastructure of Th HA cells and the distribution of FcαR on these cell lines were determined by colloidal gold (CG) immunoelectron microscopy (IEM). When Th HA cells were incubated with purified mouse IgA followed by CG-labeled anti-IgA, an even pattern of CG was distributed on the cell membrane. To ensure that binding occurred through FcαR, Th HA cells were mixed with MOPC 315 IgA anti-DNP, followed by staining with CG-labeled TNP-human serum albumin. This resulted in an identical pattern of gold particle distribution, confirming expression of FCαR on Th HA cells. No FcμR or FCγ1R were detectable on Th HA cells by IEM. Immunocytoadherence with TNP-conjugated erythrocytes confirmed that Th HA cells were FcαR+; however, no IgM or IgG rosettes were seen. When these cell lines were analyzed by flow cytometry (FACS) using IgA, IgM, or IgG1 and FITC-labeled anti-H chain-specific antibodies, 55 to 65% of cultured Th HA cells expressed FcαR, and 11 to 18% expressed FcμR; however, no Fcγ1R was detectable on Th HA cells. The use of ELISA with Th HA cells as antigen confirmed the expression of FCαR and the presence of less FcμR on these two cell lines. Solubilized membrane fractions derived from Th HA cells were tested for the presence of FcR by ELISA and for biologic function for support of IgA responses in Peyer's patch B cell cultures. Both FcαR and FcμR were detected in fractions derived from Th HA cells. Furthermore, these fractions supported in vitro IgA anti-sheep erythrocyte responses, comparable to those obtained with Th HA cell culture supernatants containing IBFα. These studies show that Th HA cells express FcαR with less FcμR, and the solubilized form of FcαR exhibits IBFα-like activity. The significance of FcR expression by Th cell clones and cell lines and the relationship of soluble FcαR and IBFα for IgA response regulation are discussed.