The response of human lymphocytes to T lectins was shown to be dependent on and regulated by THF (thymus humoral factor). When human lymphocytes were stimulated with T lectins in the presence of dialyzed human plasma (DHP) which probably contains a low thymic hormone concentration, we observed a reduced response to PHA and Con A, compared to that observed in the presence of whole human plasma (WHP). This reduced reactivity to T lectins in the presence of DHP was restored by the addition of plasma dialyzates or THF. On the contrary, addition of THF to cultures in the presence of WHP caused a significant reduction in reactivity to T lectins, suggesting that enhancement or reduction of the response of human lymphocytes to T lectins by THF depends on the endogenous content of the thymic hormone in the plasma. From the present data it is suggested that the response of human cells to PHA behaves differently from that to Con A, as reflected in their dependence on THF. Although the response of human cells to Con A is increased by THF only in the presence of DHP, the response to PHA is also enhanced by THF in the presence of low WHP concentrations of even in the absence of WHP.