The ability of tumor cells to initiate coagulation, platelet aggregation and subsequent thrombotic alterations is believed to facilitate metastatic process. The mechanisms by which tumor cells develop thrombotic events in malignancy are discussed. We have examined the procoagulant activity and the modifications of rheologic platelet functions induced by 3 human mammary tumor cell lines (MCF-7, ZR-75-1, BT-20) using a laser-thromborheometer. According to experimental conditions, these 3 tumor cell lines aggregate platelets via a thrombin-dependent mechanism. Moreover, MCF-7 cells also induce ADP-like platelet aggregation. These data suggest the existence of several mechanisms of mammary tumor cell-induced platelet aggregation.