Platelets have been hypothesized to contribute to tumor cell metastasis, but the underlying mechanism(s) remain unknown. We demonstrate here that one mechanism whereby platelets may facilitate metastasis is by potentiating tumor cell-induced endothelial cell retraction, a prerequisite for the extravasation of most tumor cell types. The integrity of cultured microvascular endothelial cell (CD3 cells) monolayers was perturbed by 12[S]-hydroxyeicosatetraenoic acid (12(S)-HETE), a lipoxygenase metabolite of arachidonic acid, as well as by tumor cells (i.e., Lewis lung carcinoma cells or 3LL). 3LL cells induced a concentration- and time-dependent retraction of the CD3 monolayers, as assessed by quantitative binding assays as well as by phase-contrast microscopy. In contrast, normal murine fibroblasts (3T3) did not induce endothelial cell retraction. 3LL cell-induced endothelial cell retraction was potentiated, in a dose- and time-dependent manner, by homologous murine platelets while platelets alone did not induce endothelial cell retraction. Platelet-enhanced, tumor cell-induced endothelial cell retraction was inhibited by treating either tumor cells or platelets with the lipoxygenase inhibitors nordihydroguaiaretic acid or N-benzyl-N-hydroxy-5-phenylpentanamide (BHPP) as well as by PGI₂ or its analogs iloprost and ZK96.480 (cicaprost), but not by the cyclooxygenase inhibitor aspirin (ASA). Tumor cells, upon adhesion to endothelium, initiated 12(S)-HETE biosynthesis, which was inhibited by pretreating tumor cells with BHPP but not with ASA. Additionally, 12(S)-HETE biosynthesis during tumor cell-endothelial cell adhesion was significantly enhanced by the addition of homologous platelets. Collectively, these results suggest that tumor cell-platelet-endothelial cell interactions lead to enhanced biosynthesis of 12(S)-HETE by tumor cells and/or platelets, which in turn induces endothelial cell retraction, thus facilitating tumor cell extravasation and metastasis.