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 (IBF alpha) which regulates antigen-dependent IgA responses. The ultrastructure of Th HA cells and the distribution of Fc alpha R 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 Fc alpha R, 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 Fc alpha R on Th HA cells. No Fc mu R or Fc gamma 1R were detectable on Th HA cells by IEM. Immunocytoadherence with TNP-conjugated erythrocytes confirmed that Th HA cells were Fc alpha R+; 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 Fc alpha R, and 11 to 18% expressed Fc mu R; however, no Fc gamma 1R was detectable on Th HA cells. The use of ELISA with Th HA cells as antigen confirmed the expression of Fc alpha R and the presence of less Fc mu R 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 Fc alpha R and Fc mu R 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 IBF alpha. These studies show that Th HA cells express Fc alpha R with less Fc mu R, and the solubilized form of Fc alpha R exhibits IBF alpha-like activity. The significance of FcR expression by Th cell clones and cell lines and the relationship of soluble Fc alpha R and IBF alpha for IgA response regulation are discussed.