Immunization with increasing doses of SRBC, in excess of 10(8), results in a progressive decline in the anti-SRBC PFC response. This hyporesponsive state is antigen specific and is reflected in a decrease of both T helper and B antibody-forming activity. We asked whether the apparent defect of T helper activity reflected a) an absence of alphaSRBC helper T cell activity, or b) the presence of SRBC-specific suppressor T cells within the hyporesponsive population. Our results indicate that at least a portion of hyporesponsiveness noted after antigen exposure to large doses of antigen can be ascribed to specific suppressor T cell activation. Fractionation of the suppressive T cell population using Ly antiserum showed that specific suppressive activity was mediated by a subclass of T cells (Ly2+), distinct from that committed to express helper function (Ly1).