Abstract Zinc transferrin, when added to serum-free cultures of phytohemagglutinin-stimulated human lymphocytes, causes an increase in deoxyribonucleic acid synthesis over that seen with phytohemagglutinin alone, as judged by the uptake of tritiated thymidine. This effect is not seen with zinc acetate, zinc albumin, or zinc ovotransferrin. Zinc transferrin also has a similar effect on ribonucleic acid synthesis. Furthermore, transferrin-bound zinc is specifically taken up by stimulated lymphocytes, maximal uptake occurring approximately 14 hr after the addition of phytohemagglutinin. These results indicate a function for serum transferrin in zinc metabolism, and, moreover, a role for the zinc transferrin complex in lymphocyte metabolism.