Cytoplasmic Ca2+ mobilization in human platelets triggered by a CD 9 monoclonal antibody, TP82, was monitored by the Ca2(+)-sensitive photoprotein, aequorin and Ca2(+)-sensitive fluorophores, fura 2 and quin 2. Aequorin-indicated Ca2+ values were proportional to the concentration of TP82, which was in inverse proportion to the lag time before the onset of platelet aggregation and serotonin release. When fura 2 was used as a Ca2+ indicator, above a threshold concentration, the TP82-induced intracellular Ca2+ value remained unchanged even with increasing concentration. The findings obtained with quin 2 were compatible with the fact that the TP82-induced intracellular Ca2+ increase was largely dependent on the secondary effect of thromboxane A2. These findings may be clues to explain the marked difference in the Ca2(+)-response characteristics between the fluorescent indicators and aequorin as well as the properties of TP82-induced platelet activation.