Although the standard assays for reactive oxygen species have been based on the measurement of those released into the extracellular environment, the microbicidal capacity to the engulfed microorganisms is mainly dependent on those released into the intracellular environment, such as phagosomes. We studied intracellular oxidative activities of individual phagocytes by dichlorofluorescein (DCFH) oxidation assay to investigate the relationship between the reactive oxygen species released intracellularly and the impaired microbicidal capacity in diabetic patients. Time courses of intracellular production of hydrogen peroxide by polymorphonuclear leucocytes (PMNL) and monocyies were observed at the resting condition and after the stimulation with phorbol myristale acetate (PMA: 160 nm) by flow cytometry. Thirty-four patients with non-insulin-dependent diabetes mellitus (NIDDM) and 23 age-matched healthy volunteers were subjected to the studies. PMNL from patients with NIDDM showed a significantly decreased capacity to produce hydrogen peroxide after the stimulation (P < 0·05 at 15 min, P < 0·01 at 30 and 45 min). By contrast, inlracellular hydrogen peroxide production by monocytes at the resting condition and an early stimulatory phase (8 min after the stimulation) was significantly (P< 0·01) enhanced in patients with NIDDM compared with that in controls. Both the changes of inlracellular hydrogen peroxide production observed in PMNL and monocytes from patients with NIDDM were in association with an increased haemoglobin Alc level in crythrocytes. but did not relate to total cholesterol and triglyceride levels in the serum. The possible mechanisms of these dissociated changes in hydrogen peroxide producing capacity of phagocytes from patients with NIDDM are discussed.