Using the two lectins from lima bean, we have tested the model for mitogenic stimulation of lymphocytes proposed by Prujansky et al. (Prujansky, A., Ravid, A., and Sharon, N. (1978) Biochim. Biophys. Acta 508, 137-146). The lectins used, a tetramer with two saccharide-binding sites and an octamer with four binding sites, are specific for N-acetyl-D-galactosamine. Our results show that cooperative binding may not be a prerequisite for mitogenicity of all lectins. We found that neither the weakly mitogenic tetramer nor the potently mitogenic octamer bound cooperatively to bovine lymphocytes. The strong mitogen bound with a higher affinity than the weak mitogen and fewer mitogen molecules bound to the lymphocyte surface at saturation. Competitive binding experiments indicated that both lectins bound to the same receptors. Our results suggest that the mitogenic lectin is able to bind and cross-link more membrane receptors. We have also studied the binding of the lima bean lectins to human red blood cells of types A, AB, B, and O. Both lectins bound cooperatively to type A and type AB cells and our data indicate that the lima bean lectins bind predominantly to the type A determinant.