Concanavalin A-mediated stimulation of 32P-phosphate incorporation into phospholipids of human peripheral lymphocytes is comparatively studied in normal and phosphate-depleted media. In the phosphate-depleted medium, 2 hours after the start of cell activation, the stimulation sharply decreases for phosphatidylinositol (6.5-fold) and for phosphatidylcholine (in the latter case, the stimulation is even replaced by a slight inhibition of the incorporation). These results must be related to the rate-limiting effect of inorganic phosphate on ATP formation and thus on phospholipid synthesis, an effect which may be particularly pronounced when there is both phosphate depletion and cell activation.