The induction of deoxyribonucleoside triphosphate pools was studied in phytohemagglutinin-stimulated human lymphocytes in the presence of metabolic inhibitors. The dATP pool was completely inhibited in cells treated with hydroxyurea, in contrast to the dTTP pool. However, 1-formyl-isoquinoline thiosemicarbazone inhibited the formation of these pools equally. During approximately 3 hr of treatment of stimulated cells with hydroxyurea, the dATP, dGTP and dCTP pools were depleted to the base levels observed in the cells before the pools were induced. The base level of the dTTP pool was achieved only in the presence of 5-fluorodeoxyuridine, but the inhibition was completely prevented by addition of thymidine. It is suggested that, when resting lymphocytes were stimulated to enter the growth cycle, the formation of deoxyribonucleoside triphosphates in the early transformation was due to the de novo pathway.