Concanavalin A (Con A) increased the thymidine incorporation levels of normal, human, peripheral blood lymphocytes cultivated without serum. The marked increases were maximum at 72 hours and proportional to the concentration of Con A. The maximum responses induced by low concentration (0.5–1.0 μg/ml)were 23- to 66-fold greater than their controls, whereas 50–100 μg/ml tended to be cytotoxic. Human lymphoid cell lines RPMI 4098, 1788, and 7249, cultured without serum supplementation, maintained high levels of spontaneous thymidine incorporation and viability up to 3 days. With low Con A concentrations, the thymidine incorporation levels of cell lines 1788 and 7249 were increased above their spontaneous incorporation levels. These stimulation responses to Con A were comparable to those of normal peripheral cells by several criteria. Higher Con A concentrations were cytotoxic for all 3 cell lines as well as for normal ones. The stimulatory and cytotoxic effects of Con A on these 2 lymphoid populations were inhibited by α-methyl-d-mannoside, an inhibitor of Con A binding to glycoproteins. The Con A activation and destruction of normal peripheral lymphocytes and lymphoid cell lines may have occurred through analogous mechanisms.—J Natl Cancer Inst 49: 935–941, 1972.