The thrombospondins (TSPs) are a growing family of cell surface and extracellular matrix molecules composed of multiple repeating elements. Thrombospondin 3 is a recently described member that possesses the calcium binding Type 3 repeats, has 4 epidermal growth factor receptor-like Type 2 repeats, a complete absence of the complement-like Type 1 repeats, and a distinct N terminus that has no significant homology to the other TSPs. Metabolic labeling and immunoprecipitation analysis of cells transfected with a TSP3 expression vector revealed it to be an oligomeric heparin binding protein present in both the cell layer and medium. Finally, a combination of in situ hybridization and immunocytochemistry demonstrated TSP3 to be expressed in a temporal and spatial manner during murine embryogenesis, especially in the gut, cartilage, lung, and central nervous system.