A thymocyte growth peptide (TGP), recruiting immature thymocytes into the S phase without affecting immunologically mature thymocytes or peripheral T cells, has previously been purified from bovine and rodent thymus. We now report the isolation of a corresponding human activity. Aqueous extracts of human thymus were found to stimulate the DNA synthesis of human thymocytes in vitro. Homogenates of suspended thymocytes displayed higher activity than extracts of intact thymus, whereas extracts of thymic stroma depleted of thymocytes contained much weaker activity. The results indicate the existence of a human TGP originating from the thymocytes and not from the stroma, implying an autocrine or paracrine growth regulation. Human TGP was purified from a formic acid extract of human thymus and found to have a molecular ratio between 1,000 and 2,000 and an isoelectric point of 4.5-4.9. The results demonstrate the existence of a human TGP similar but not identical to bovine TGP and it is proposed that this peptide acts as a progression factor for the intensely proliferating immature cortical thymocytes.