An unusual mechanism has been demonstrated for the in vitro proaggregating interaction between human platelets and human epidermoid carcinoma A431 cells. A431 cells induce platelet aggregation in a dodependent manner, depending on the rate of ADP release from tumour cells, which occurs in the presence not only of platelet rich plasma (PRP) but also of platelet poor plasma (PPP) or serum. This ADP release appears to be correlated to C3cleavage and binding of C3cto the A431 cell membrane. The interaction between A431 cells and PRP is characterized by typical morphological changes of A431 cells, leading to formation of mixed aggregates showing long projections of tumour cells deeply penetrating into the aggregate. These features, lacking in the presence of gel-filtered platelets (GFP), and reduced in the presence of thrombin degranulated platelets (TDP), are inhibited by cytochalasin and RGDS. The same activation of A431 cell cytoskeleton is induced by PDGF, but not by ADP or thromboxane receptor agonist U46619 or TGFP. These findings suggest a cooperative mechanism of tumour cell platelet interaction, in which a complementdependent ADP release from A431 cells induces platelet degranulation, PDGF release and aggregation. PDGF may induce in A431 cells Ca2+influx, cytoskeleton activation and changes in exposition of surface adhesion molecules, while fibrinogen binding causes mixed tumour cell-platelet aggregates to form.