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SummaryTwo signals of chemiluminescence are observed when platelets are exposed to arachidonic acid in the presence of luminol. Three groups of agents interfere with these luminescence responses. Inhibitors of cyclooxygenase, known to augment the turnover of arachidonic acid by lipoxygenase, inhibit the first and enhance the second signal of luminescence.Sodium azide, diamide, NEM, and the endoperoxide analogues U 44069 and U 46619 interfere with the second luminescence signal but not with the first one nor with the generation of MDA. These agents may represent selective inhibitors of the lipoxygenase pathway.Phenidone, nordihydroguaiaretic acid, quercetin, silybin, phenylthiazolyl-thiourea, and aminotriazole inhibit both luminescence signals promoted by arachidonic acid.Measurement of luminescence may provide a tool to follow the time course of arachidonic-acid turnover by prostaglandin synthetase and lipoxygenase in whole cells.