Cells from the spleens of “normal” swine, which were pretreated with pronase to remove surface membrane-bound immunoglobulin, gave an enhanced hemolytic plaque-forming cell response to sheep red blood cells in vitro in comparison with untreated controls. The enhancement could be abrogated by preincubating pronase-treated spleen cells in preparations containing antibody to sheep red blood cells. This effect was demonstrated by autologous sera, immune sera, and all three known classes of porcine serum immunoglobulins, including IgM, IgA, and IgG and could be removed by absorption with sheep red blood cells. Surface membrane-bound antibody exerted its effect by binding to the nonadherent cell population. The response of normal spleen cells was unaffected by antibody treatment. Pronase-treatment was not mitogenic, did not function as a polyclonal B cell activator, and did not selectively eliminate T or B cells. The results indicate that removal of antibody from the surface of lymphoid cells enhanced the humoral immune response in vitro and confirm that membrane-bound antibody can inhibit response to antigen.