Peptides from both the nervous and endocrine system have been shown to influence immune functions. This study describes the stimulatory effect of bombesin on natural killer cell activity of peripheral blood mononuclear cells. The stimulation of cytotoxicity by bombesin in vivo was much higher than found in vitro. In vitro studies with bombesin and gastrin revealed that the stimulatory effect of bombesin in vivo can for a major part be attributed to other stimulatory mediators which are released by BBS. These results indicate that neuropeptide release might rapidly interfere, both directly and indirectly, with natural killer activity of peripheral blood mononuclear cells.