AbstractLectin mixtures from 10 samples of Phaseolus coccineus and Phaseolus vulgaris beans were used as immunosuppressive agents in the sheep red blood cell (SRBC) antibody response of the mouse. Some bean samples contain lectins capable of inducing several times better suppression of humoral immunity than concanavalin A and phytohemagglutinin tested before in the same immune system.High dilutions of the 10 bean extracts were shown to agglutinate mouse, rabbit and sheep erythrocytes in vitro but the titers and “specificities” did not correlate with the immunosuppressive potency and selectivity displayed by the same extracts in mice.Two immunosuppressive bean extracts, assayed in lymphocyte cultures, were found to stimulate DNA synthesis of murine spleen T cells. This interaction appears to be a necessary step for the reduction of antibody synthesis to SRBC in the whole animal.