We have studied in a homologous system the effect on different platelet functions of cells isolated from 26 human tumor tissues (11 breast carcinomas, 11 colon carcinomas, 2 pancreatic carcinomas, 1 gastric carcinoma and 1 esophageal carcinoma). Tumor cells (10(5)/ml) significantly increased platelet adhesion to glass beads; they were also found to possess a potent platelet aggregating activity and aggregation was accompanied by significant release of ATP and platelet derived growth factor (PDGF) and by production of TXB(2). Preincubation of platelets with a low concentration (1 µM) of indobufen, a cyclooxygenase inhibitor, significantly reduced tumor cell induced TXB(2) production and ATP release, while the other platelet functions were not modified. Higher concentrations of the drug (10 or 100 µM) were also able to inhibit tumor cell-induced platelet aggregation and PDGF release, while platelet adhesion to glass beads was unchanged even at these doses. Finally, preincubation of neoplastic cells with indobufen (400µM) had no effect on their ability to induce platelet aggregation, TXB(2) production and ATP release. These data demonstrate that cyclooxygenase blockade in platelets has different effects on several platelet functions activated by the tumor cells that were investigated.