Flow cytometry was used to study phagocytic function and release of reactive oxygen intermediates (ROI) following phagocytosis by granulocytes in 14 patients (six female, eight male) with gram-negative septicaemia prior to, during, and after therapy compared with a group of healthy controls. Phagocytic capacity was assessed by measuring uptake of fluorescein isothiocyanate (FITC)-labelled bacteria. Reactive oxygen generation after phagocytosis was measured by the quantification of dihydrorhodamine 123 converted to rhodamine 123 intracellulary. Compared with results in healthy controls granulocytes of septicaemic patients exhibited a decreased capacity to phagocytize Escherichia coli and to generate reactive oxygen products. Both phagocytosis and ROI production increased after initiation of therapy and normalized within 7 days of treatment. The results suggest that granulocytes do not only participate in, but are also a target of, the septic host inflammatory response.