Since the first report in 1983, blood platelet reactivity from patients with immediate hypersensitivity reactions, and particularly allergic asthma and aspirin-sensitive asthma, has been evaluated by the release of cytotoxic mediators able to induce the death of parasite larvae, and by the generation of oxygen derived free radicals, measurable by chemiluminescence. We demonstrate here that platelets are able to reduce MTT tetrazolium salt proportionally to the stimulation level in two models of platelet triggering, one IgE-dependent and one aspirin-dependent. This method correlated significantly with the cytotoxicity assay of platelet stimulation (r = 0.965, p < 10(-5) for IgE-dependent stimulation; r = 0.723, p < 10(-4) for aspirin-dependent stimulation). The MTT colorimetric assay should complement or possibly replace previous methods of assessing platelet activation which were of limited use allowing broader investigations on the involvement of platelets in immune processes and in allergic or pseudoallergic reactions.