The thrombospondins are a family of extracellular calcium binding proteins that are involved in cell proliferation, adhesion, and migration. We have sequenced full-length human thrombospondin-4 and characterized the recombinant protein. In contrast to Xenopus laevis thrombospondin-4, the human protein contains an RGD cell binding sequence in the third type 3 repeat. Transfection of mouse NIH3T3 fibroblasts or C2C12 myoblasts with a full-length human thrombospondin-4 cDNA results in the expression of a polypeptide with a reduced molecular weight of 140,000. In the absence of reducing agent, the expressed protein has an apparent molecular weight of 550,000. Recombinant thrombospondin-4 has been purified from the culture supernatant by heparin-Sepharose and anti-thrombospondin-4 antibody-Affi-gel affinity chromatography. Electron microscopy indicates that thrombospondin-4 is composed of five subunits with globular domains at each end. The observation of a calcium-dependent change in the electron microscopic appearance of thrombospondin-4 is consistent with limited tryptic digestion data that indicate that thrombospondin-4 is resistant to digestion in the presence of calcium. These data indicate that thrombospondin-4 is a pentameric protein that binds to heparin and calcium.