In the first familial case of gray platelet syndrome (GPS) reported in Japan in which the subjects showed platelet release abnormalities, we investigated the relationship between intracellular Ca++ mobilization in platelets and marked morphological abnormalities of the DTS. The subjects for the Aequorin assay were 12 controls and 11 GPS cases, whereas the subjects for the Fura-2 assay were 10 controls and 6 GPS cases. In order to discriminate between Ca++ influx from outside of the cells and Ca++ mobilization from DTS within the cells, the experiments were conducted under two conditions; one in the presence of 1 mM Ca++ in the external fluid, and the other with the addition of 2 mM EDTA as a Ca++ chelator. Stimulation by A-23187 in the presence of 1 mM Ca++ in the external fluid caused 2 peaks or shoulder formation; that is, normal cases showed 1 peak at all concentrations of A-23187 tested, whereas GPS showed 2 peaks or shoulder formation in 7 of the 11 cases and conspicuously good reproducibility in each case. These facts indicated that it took time for the stimulus to reach the inside of the DTS, which showed marked morphological abnormalities. During stimulation by 1.0 U/ml thrombin under the same conditions, the GPS exhibited 1 peak with a wide skirt pattern, compared with the control. In one case of GPS, which revealed one peak by thrombin and 2 peaks by A-23187 in the presence of 1 mM Ca++, 2 peaks were also noted by thrombin and the luminescence peak become lower, when Ca++ was chelated by EGTA, using the Aequorin method.(ABSTRACT TRUNCATED AT 250 WORDS)