The relationship between the production of a T cell factor having affinity for IgA (IgA-binding factor(s); IgA BF) and the expression of Fc receptors specific for IgA (Fc alpha R) was studied by using murine spleen cells activated with concanavalin A (Con A blasts). Fc alpha R was detected by the cytophilic binding of anti-TNP murine IgA myeloma protein (MOPC 315 IgA) to Con A blasts as determined by an indirect rosette method with trinitrophenylated sheep red blood cells (TNP-SRBC). After 18 hr preculture with IgA, Fc alpha R was expressed on 15 to 20% of Con A blasts, which released IgA BF suppressing the in vitro IgA synthesis of the spleen cells stimulated with pokeweed mitogen (PWM). Without preculture with IgA, there was neither induction of Fc alpha R nor the production of IgA BF from Con A blasts. Fc alpha R was not induced on Con A blasts by IgA if Fc gamma R(+) cells were depleted from the blasts by rosetting with SRBC sensitized with rabbit IgG antibody (EA gamma). Even after preculture with IgA, the suppressive IgA BF was undetectable in the culture supernatant of Con A blasts depleted of the Fc gamma R(+) cell population. By using a double rosette method with EA gamma and trinitrophenylated quail red blood cells, Fc alpha R proved to be co-expressed on Fc gamma R(+) precursor T cells in the Con A blasts. The results suggested that both Fc gamma R and Fc alpha R could be co-expressed on Con A blasts, as is the case with T2D4 Fc gamma R(+), Fc alpha R(+) T hybridoma cells, which are known to produce IgG-binding factor(s) (IgG BF) and IgA BF. The relationship between Fc gamma R and Fc alpha R on a single cell was studied by using monoclonal anti-Fc gamma R antibody (2. 4G2 ). The reactivity of 2. 4G2 antibody with T cell Fc gamma R was proved by the inhibition of EA gamma rosette formation by Con A blasts or T2D4 cells. The addition of 2. 4G2 monoclonal antibody, however, did not affect the induction of Fc alpha R on Con A blasts by IgA. Furthermore, the binding of IgA to Fc alpha R already expressed on L5178Y T lymphoma cell line cells was not inhibited by the monoclonal antibody. The results confirmed that Fc alpha R are distinct from Fc gamma R co-expressed on the same Con A blasts, and that the expression of Fc alpha R on Fc gamma R(+) T cells and their production of suppressive IgA BF may be induced by the binding of IgA to Fc alpha R.