Thymopoietin (TP) was originally isolated as a 5-kDa 49-aa protein from bovine thymus in studies of the effects of thymic extracts on neuromuscular transmission and was subsequently observed to affect T-cell differentiation and function. We now report the isolation of cDNA clones for three alternatively spliced mRNAs that encode three distinct human T-cell TPs. Proteins encoded by these mRNAs, which we have named TP alpha (75 kDa), TP beta (51 kDa), and TP gamma (39 kDa), contain identical N-terminal regions, including sequences nearly identical to that of the originally isolated 49-aa protein, but divergent C-terminal regions. TP mRNAs are expressed in many tissues, most abundantly in adult thymus and fetal liver of the tissues so far examined. Distinct structural domains and functional motifs in TPs alpha, beta, and gamma suggest that the proteins have unique functions and may be directed to distinct subcellular compartments.