Binding and mitogenic characteristics of the 2 Phaseolus lunatus (lima bean) lectins have been examined with human peripheral blood lymphocytes. Chemical cross-linking of the nonmitogenic lima bean lectin produced a species that stimulated human lymphocytes as well as or better than the mitogenic lima bean lectin, which is a T lymphocyte mitogen with requirement for monocyte participation. The maximal stimulation and the dose response to the cross-linked lima bean lectin did not significantly differ from that observed with the mitogenic lectin. We have used fluorescein-labeled lima bean lectins to show that both lectins share mutually exclusive binding sites on lymphocytes. Our results strongly support the concept that multiple valence of lectins is important in inducing mitogenesis. Both the mitogenic and nonmitogenic lectins demonstrated selective binding by labeling only 70% od human peripheral blood lymphocytes. The fraction not binding lectin is a population of T lymphocytes. Competitive binding studies with the lima bean lectins and other N-acetyl-D-galactosamine-specific lectins suggest that the cell surface receptors for these various lectins may be quite different. We have also studied the response of human lymphocytes to the lima bean lectins and soybean agglutinin after neuraminidase treatment. As previously demonstrated (22), neuraminidase treatment of the cells drastically altered the binding and mitogenic response to soybean agglutinin. Lima bean lectin binding to the treated cells was also markedly increased, but the mitogenic response was essentially unaffected.