1. Neutrophils play a role in the pathogenesis of ulcerative colitis. Most of the patients with ulcerative colitis are non-smokers. 2. Neutrophil function was examined in healthy males, five smokers and five non-smokers, measuring production of oxygen free radicals by chemiluminescence. 3. The chemiluminescence was examined in the presence of nicotine and its main metabolite cotinine in various concentrations. 4. 10 mM nicotine inhibited production of oxygen free radicals (90.2% +/- 11.6) compared with controls (P less than 0.001), as did the same concentration of cotinine (58.9% +/- 34.2), (P less than 0.05). 5. 1 mM solutions had no effect and 3 mM nicotine was required to achieve 50% inhibition. 6. Smokers and non-smokers did not differ. 7. Since plasma concentrations of nicotine after a cigarette are only between 5 and 15 mM, the observed effects of nicotine and cotinine in much higher pharmacological concentrations in this system are unlikely to be of clinical relevance in ulcerative colitis.