SW-480 cells, derived from a primary human colon adenocarcinoma, caused dose-dependent platelet aggregation in heparinized human platelet-rich plasma. SW-480 tumour cell-induced platelet aggregation (TCIPA) was completely inhibited by hirudin (5 U/ml) but unaffected by apyrase (10 U/ml). This TCIPA was also unaffected by cysteine proteinase inhibition with E-64 (10 microM) but was limited by cell pretreatment with phospholipase A2. SW-480 cell suspension caused marked dose-dependent decreases in plasma recalcification times using normal, factor VIII-deficient and factor IX-deficient human plasma. This effect was potentiated with cell lysates but inhibited in intact cells pretreated with sphingosine. SW-480 cell suspension did not affect the recalcification time of factor VII-deficient plasma. Moreover, monoclonal antibody against human tissue factor completely abolished SW-480 TCIPA. Taken together, these data suggest that SW-480 TCIPA arises from SW-480 tissue factor activity expression. Trigramin and rhodostomin, RGD-containing snake venom peptides, which antagonize the binding of fibrinogen to platelet membrane glycogen IIb/IIIa, prevented SW-480 TCIPA. Likewise, synthetic peptide GRGDS as well as monoclonal antibodies against platelet membrane glycoprotein IIb/IIIa and Ib prevent SW-480 TCIPA, which was unaffected by control peptide GRGES. On a molar basis, trigramin (IC50 0.09 microM) and rhodostomin (IC50 0.03 microM) were about 6000 and 18,000 times, respectively, more potent than GRGDS (IC50 0.56 mM).