OBJECTIVE To investigate the immunosuppressive effect of peritoneal fluid and follicular fluid on both natural killer-mediated cytotoxicity and phytohemagglutinin-induced lymphocyte proliferation. METHODS The peritoneal fluid of women with endometriosis was compared to both fertile and infertile control fluids. Lymphocytes were pretreated for 2 or 20 hours with peritoneal or follicular fluids, and their cytotoxicity toward K562 tumor cells was measured. We also investigated the phytohemagglutinin-induced stimulation of lymphocytes cocultured with peritoneal or follicular fluid. RESULTS Peritoneal fluid from women with endometriosis had a significantly greater immunosuppressive effect on natural killer-mediated cytotoxicity and on phytohemagglutinin stimulation of lymphocytes compared to peritoneal fluid of fertile women without endometriosis (P < .01 and P < .05, respectively). Using the peritoneal fluid of infertile women without endometriosis, these differences were significant only when compared to women with severe endometriosis. Inhibition of the natural killer activity increased when the incubation period was prolonged from 2 to 20 hours (P < .04). There was no correlation between the immunosuppressive effect of peritoneal fluid and the volume of peritoneal fluid, the day of the menstrual period, or estradiol, progesterone, prostaglandin E2, or prostaglandin F2 alpha levels. In peritoneal fluid, the factor responsible for inhibition of natural killer activity was not removed with charcoal treatment. In follicular fluid, on the other hand, the inhibition of natural killer activity decreased significantly after treatment with charcoal. CONCLUSIONS Natural killer activity is suppressed by the peritoneal fluid of women with severe endometriosis; this may be important in the pathogenesis of endometriosis. The factor responsible for the inhibition of natural killer activity in peritoneal fluid is different from that in follicular fluid.