The production of reactive oxygen intermediates (ROI) in compound 48/80- or calcium ionophore A 23187- activated pleural or peritoneal mast cells was monitored using flow cytometry and the fluorescence indicator dihydrorhodamine 123 (a derivate of rhodamine 123). Mast cell degranulation and ROI production were estimated by flow cytometric fluorescence and light-scatter analysis. In addition, activation was monitored by spectrofluorimetric measurement of histamine secretion from the cells. We used flow cytometric measurement of narrow and wide angle scatter and fluorescence intensity to distinguish between degranulated and resting mast cells during activation and to monitor the two populations separately. Both stimuli induced a dose-dependent elevation of ROI production in mast cells which was accompanied by cell degranulation and histamine secretion. These alterations were completely abolished by preincubation of the mast cells with diethyldithiocarbamate (DTC). Using low concentrations of DTC partial decrease of degranulation and histamine secretion was observed while ROI production was blocked. D-mannitol increased ROI production in stimulated mast cells without a marked effect on degranulation or histamine secretion. The data suggest that mast cell degranulation (histamine secretion) and ROI production are independent processes.