Thiazole orange (TO), a fluorescent dye originally synthesized for reticulocyte analysis, is characterized by a large fluorescence enhancement and high quantum yield on binding to nucleic acids, particularly RNA. In addition, the dye readily permeates live cell membranes. We applied TO staining, followed by fluorescence-activated flow cytometric analysis, to platelets in whole blood samples from hematologically normal subjects and patients with various quantitative platelet disorders. The percentage of TO-positive platelets in 50 control subjects was 8.6 +/- 2.8% (mean +/- SD) ranging from 2.8% to 15.8%. In 21 thrombocytopenic patients whose bone marrow contained normal to increased numbers of megakaryocytes, the percentage of fluorescently labeled platelets was significantly elevated (P less than .0001) to 26.9 +/- 10.9% (range, 13.3% to 57.1%). In contrast, the proportion of positively stained platelets in 23 patients with thrombocytopenia due to impaired platelet production (various conditions with reduced marrow megakaryocytes) did not significantly differ from the controls, whereas the absolute counts of TO-positive platelets were significantly lowered (P less than .0001). Differences in the distributions of the percentage values as well as of the absolute counts for TO-positive platelets between the two patient groups were again highly significant (P less than .0001). Both the sensitivity and the specificity of this method in distinguishing between these categories of thrombocytopenia were greater than or equal to 95%. We conclude that flow cytometric analysis of platelets after staining with TO is a sensitive and specific test that rapidly provides information on the thrombopoietic activity in thrombocytopenic disorders. Our data further suggest that increased amounts of residual RNA characterize platelets released under conditions of “stress thrombopoiesis.”