The diverse and occasionally conflicting properties described for the extracellular, cell surface-associated protein thrombospondin (TSP) have raised the possibility that functionally distinct forms of the protein exist in the same organism. We have isolated and characterized a partial cDNA clone for mouse TSP that is clearly homologous to, but distinct from, the coding sequence for mouse TSP deduced from a mouse genomic clone (Bornstein, P., Alfi, D., Devarayalu, L., Framson, P., and Li, P. (1990) J. Biol. Chem. 265, 16691-16698). This second TSP, which we term thrombospondin 2, is the product of a separate gene (Thbs2) and is expressed in a variety of mouse tissues in a pattern that differs from that for TSP1. Based on their translated amino acid sequences, it seems likely that TSP1 and TSP2 will be found to have both common and unique properties and that the functional consequences of TSP production will reflect the ratio of the levels of these two related proteins.