Serotonin release from human platelets. Human platelets (1 × 10⁸/mL) were stimulated with the mouse MoAb to human FcɛRI  chain (CRA1 and CRA2, each 10 μg/mL), or with mouse MoAb to human FcγRII/CD32 (IV.3, 10 μg/mL), or with thrombin (10 U/mL). The levels of released serotonin were measured as radioactivity, in the supernatants and the percent release was estimated as described in Materials and Methods. Spontaneous release was always < 1% of the total serotonin content. Results are shown as mean ±SD (n = 3). *P < .01.