Recent investigation of a biologically active synthetic polymer, NED 137, has demonstrated its ability to induce a B cell differentiation response to certain antigens, even in the presence of T cell depletion. In this report, the effect of T cell depletion combined with NED 137, on skin allografting, is explored in the Lewis strain rat. Animals were T cell-depleted by thymectomy, total body irradiation, and syngeneic marrow repopulation. They were then challenged with skin allografts ± NED 137 treatment. Graft survival was significantly prolonged in the T-depleted rats regardless of treatment with NED 137. The drug did not increase the immune response to donor antigen as measured by in vivo lysis of 51Cr-labeled cells. Immunization with heterologous erythrocytes produced a low level of differentiation of IgM-producing cells in the T cell-depleted skin allografted group, but in contrast the T-depleted NED 137-treated rats had a normal response to immunization. These data suggest that selective stimulation of the humoral component of the immune response is feasible at a time when T cell-mediated function has been radically suppressed, without producing adverse effects on allograft survival.