To study peritoneal fluid (PF) prostaglandin (PG) in infertile women, 6-keto-PGF1 alpha (a breakdown product of prostacyclin) and thromboxane B2 (T X B2) (a metabolite of T X A2) were assayed with radio-immunoassays from PF samples collected at laparascopy from patients with endometriosis (n = 29), unexplained infertility (n = 13) and from women with normal pelvic organs (n = 25). The concentrations of 6-keto-PGF1 alpha and T X B2 in PF were increased (p less than 0.05) in endometriosis and unexplained infertility, as compared with the corresponding levels in the controls. In patients with endometriosis, both 6-keto-PGF1 alpha and T X B2 increases were related to the severity of the disease. There was no relationship between 6-keto-PGF1 alpha/T X B2 in PF, and day of menstrual cycle. It is suggested that endometriotic tissue and peritoneal macrophages may contribute to these prostanoids in PF. The smooth muscle activities of PF prostacyclin and T X A2 may be involved in infertility by interfering with tubal function.