In PMN/platelet suspensions stimulated by fMLP giant mixed aggregates are formed and TxB₂ and LTC₄ are synthesized as the result of the cooperation in the arachidonic acid (AA) metabolism during cell/cell contact. PMN-derived cathepsin G induced the expression of P-selectin on platelet surface. GE12, an antibody against P-selectin, significantly reduced mixed cell aggregates. GE12 did not affect platelet aggregation induced by PMN-derived supernatants, indicating that the inhibitory effect of GE12 on mixed cell aggregation depends on inhibition of PMN/platelet adhesion. GE12 significantly reduced TxB₂ and LTC₄ production in PMN/platelet mixed cell suspensions stimulated by fMLP. As previously reported, synthesis of ³H-TxB₂ in ³H-AA-labeled PMN/unlabeled platelets indicates that platelets utilize ³H-AA from PMN. ³H-LTC₄ production in unlabeled PMN/³H-AA-labeled platelets indicates that bidirectional routes are utilized in this system for LTC₄ synthesis. GE12 significantly reduced ³H-TxB₂ and ³H-LTC₄ synthesis. These results show that cathepsin G released by activated PMN induces the expression of P-selectin on platelet membrane: this adhesive glycoprotein modulates cell-cell contact and transcellular metabolism of AA.