It has been recently reported that neutrophils are involved in the regulation of NK cell activity. However, the mechanism of such regulation is unclear. The present study was designed to investigate the mechanisms involved in the regulation of NK cytotoxicity by human neutrophils. The role of indomethacin, an anti‐inflammatory drug, in this interaction was studied. NK cells were purified from peripheral blood obtained from normal individuals. NK cell cytotoxicity was tested on K 562 cell line by Cr release assay, Autologous neutrophils obtained from peripheral blood were stimulated by opsonized zymosan either in the presence or absence of indomethacin. The role of neutrophil supernatant containing oxygen radicals and prostaglandins on NK cytotoxicity was examined. It was shown that supernatants from stimulated neutrophils significantly inhibited (P<0.05) the autologous NK cell cytotoxicity. The presence of indomethacin in thein vitroreaction mixture, or given orally to donors, partially or completely abolished the inhibitory effect of neutrophil supernatant. Indomethacin inhibited prostaglandin E2 release, and luminol‐enhanced, myeloperoxidase‐mediated chemiluminescence of activated PMN. Diafiltration of neutrophil supernatant showed that the inhibitory activity was present in the fraction containing molecules lower than 5,000 daltons. In conclusion, our findings indicate that down‐regulation of NK cytotoxicity is mediated by prostaglandins produced by stimulated neutrophils and possibly by oxygen r