Production of reactive oxygen species by neutrophils was analyzed by a chemiluminometric method in the presence of luminol in ten healthy donors, eight patients with gastric cancer and ten patients with gastric precancer. The neutrophil chemiluminominescence response to recombinant tumour necrosis factor (rTNF) was almost five times lower in the gastric patients when compared with healthy donors and precancer patients. The chemiluminescence response to zymosan was decreased only in the gastric cancer patients with chemiluminescence index activation (CTNF) less than 1. The observed changes of neutrophil functions are thought to result in a decrease of neutrophil cytotoxic activity in gastric cancer patients.