Thrombospondin-1 (TSP-1) is different from other components of the extracellular matrix (ECM) regarding its production and distribution. TSP-1 is considered to be released in large quantity in inflammatory sites and exogenously added TSP-1 does not bind to preformed ECM but instead binds to cells. To define the physiological role of TSP-1 in the immune system, we studied the influence of TSP-1 on the in vitro culture of T cells and antigen-presenting cells (APCs) in the presence of phytohemagglutinin. By adding soluble TSP-1 to the culture, T cell proliferation was suppressed and anti-inflammatory cytokine IL-10 secretion by APCs was enhanced. The enhanced expression of IL-10 was also demonstrated at the mRNA level by RT-PCR using multiprimer kit for cytokines. The suppression of T cell proliferation and the enhancement of IL-10 secretion with soluble TSP-1 was inhibited byadding RGDS peptide or heparin. This result indicates that the effect of soluble TSP-1 may be caused by binding to its ligand(s) on T cells and/or APCs, resulting in transducing regulatory signals to the cells or disturbing appropriate interaction between T cells and APCs. We therefore propose that TSP-1 is an immunosuppressive modulator which may play a role in inflammatory sites.