T lymphocytes bearing Fc-IgA receptors were studied for proliferative responses to mitogens and allogeneic cells, and for their ability to provide help for B cell activation in pokeweed mitogen-driven immunoglobulin production. These responses were compared with those lymphocytes bearing Fc-IgG and Fc-IgM receptors. T(A) cells were found to be generally less responsive in all functional assays than T(M) or T(G) cells. Their proliferative responses to PHA and PWM averaged 5 to 20% of the responses seen with the other T cell subsets. The T(A) cell response to mitogen was more significantly augmented by the addition of irradiated adherent accessory cells than were responses of the other T cell subpopulations. In addition, the peak response of the T(A) population to PHA and PWM occurred significantly later than the peak response of unseparated T cells or the other T cell subsets. T(A) proliferative responses to allogeneic stimulator cells in mixed lymphocyte cultures averaged approximately one-third of the response of T(M) cells, and one-half the response given by T(G) cells. Each of the T cell subsets was found to provide help for B cell activation by pokeweed mitogen, although again the T(A) cells were less active than the other T cell subsets in helping immunoglobulin production in vitro. Helper activity for each class of immunoglobulin was present in each of the T cell subsets, regardless of the specificity of the Fc receptor expressed on the T cell.