We examined the cell-mediated immune response in women with pelvic endometriosis by quantitating their T and B lymphocytes. The study was conducted on heparinized peripheral blood and peritoneal fluid. The results were compared with those in a control group of women without evidence of endometriosis. Each patient underwent laparoscopy as part of her infertility evaluation. The ability of T cells to form rosettes with sheep erythrocytes was used to estimate the percentage of total and active T cells. B lymphocytes were quantitated by erythrocyte antibody complement binding capacity. In addition, T and B lymphocytes were assayed with the Immunobead technique using specific monoclonal antibodies. The helper T cell (T4) and suppressor T cell (T8) subsets of lymphocytes were evaluated with specific monoclonal antibodies coupled to microbeads. The results demonstrated an increased number of T and B cells in peritoneal fluid and peripheral blood from patients with endometriosis as compared with controls. Furthermore, the ratio T4:T8 was significantly increased in patients with endometriosis. These results suggest a cell-mediated immune response in the presence of endometriosis.