Type XI collagen in its native fibrillar but not in soluble monomeric form mediates human platelet aggregation and release of adenosine triphosphate in a dose-dependent manner. Its action is inhibited by aspirin. Type XI collagen also increased radiolabelled phosphate incorporation into protein bands with molecular weights of 42 KDa and 22 KDa, respectively. In contract, these events were not observed in platelets incubated with type IX collagen. These results suggest that the fibrillar type XI collagen has the same ability as other types of collagen to induce human platelet aggregation.