The mitogenic effect of a number of plant agglutinins was investigated in mouse lymphocyte cultures set up with different populations of T and B cells (lymph node cells and thoracic duct cells from normal mice, cortisone-resistant thymus cells, and lymph node cells from congenitally athymic mice). Concanavalin A and three agglutinin preparations from Phaseolus vulgaris were mitogenic for T cells only. Pokeweed mitogen and Robinia pseudoacacia agglutinin stimulated both T and B cells. The mitogenicity of Ulexeuropaeus agglutinin I was restricted to B cells. Ulex europaeus agglutinin II, wheat germ agglutinin and the soybean agglutinins were not mitogenic. The lymphagglutinating and lymphocyte-stimulating activity profiles of the different lectins were unrelated.