Human melanoma cell Line, HMV-I, has been shown to induce platelet aggregation without thrombin generation. The platelet-aggregating activity of HMV-I cells increased with increasing culture periods of the cells. However, data indicate that the platelet-aggregating activity of HMV-I cells is dependent upon the cell density of the culture and does not result from the culture periods- nor cell cycle-dependent regulation. It is suggested that this effect of cell density is referred to as density-induced upregulation of platelet-aggregating activity.