With six purified plant lectins as probes of cell surface structure, human T, B, and Null lymphocytes have been examined for cell surface differences. The results of binding experiments with radioiodinated lectins indicate that these three lymphocyte populations, although functionally different, bind equivalent amounts of E- and L-PHA, lentil PHA, Con A, RCA-I, and WGA. Since these lectins are capable of binding to at least four different cell surface oligosaccharides, these data indicate that human T, B, and Null lymphocytes do not differ with respect to surface content of these carbohydrate structures. In addition, all six lectins were investigated for their ability to stimulate DNA synthesis in these human lymphocyte populations. Four of these lectins (E- and L-PHA, lentil PHA, and Con A) were mitogenic for human lymphocytes, whereas RCA-I and WGA failed to stimulate significant DNA synthesis in any lymphocyte population. When incubated with the mitogenic lectins, both B and Null cells displayed a delayed peak of DNA synthesis in comparison to T cells. Thus, despite virtually identical binding characteristics, the lectin-induced mitogenic response exhibited by B and Null cells was markedly different kinetically from that of T cells.