1. Chronic granulomatous disease (CGD) is a group of genetic disorders characterised by recurrent severe suppurative infections due to impaired microbial killing. The principal biochemical defect is an impairment in the production of reactive oxygen intermediates by phagocytes. 2. Nitric oxide (NO) is synthesised from the guanidino nitrogen atom(s) of L-arginine and has recently been proposed to be involved in defence mechanisms. The aim of this study was to investigate the involvement of the oxidative burst in the biosynthesis of NO by neutrophils and mononuclear cells from patients with CGD. 3. NO synthesis was assayed by the ability of neutrophils and mononuclear cells to inhibit thrombin-induced washed platelet aggregation while superoxide anion (O2-) production was measured spectrophotometrically by the superoxide dismutase inhibitable reduction of cytochrome c. 4. Neutrophils and mononuclear cells from patients with CGD released NO. This release was inhibited by nitro-L-arginine methyl ester but could be reversed by L-arginine. Zymosan- and PMA-induced O2- production was less than 10% as compared with healthy controls. 5. These results indicate that O2- production is not essential for NO synthesis in human leucocytes.