The purpose of this study was to find out differences in the levels of antibodies to distinct antigens in the serum of fertile versus infertile patients with and without endometriosis and to identify these antigens. Blood was collected from 61 patients undergoing laparoscopy for pelvic pain, infertility, or tubal ligation. Serum antibodies against serum antigens with apparent molecular masses of 22 kDa and 18 kDa were assessed by immunoblot analysis. Gel filtration, HPLC DEAE ion-exchange chromatography, NH2-terminal sequencing, and double immunodiffusion were used to characterize and identify these proteins. The relative amount of antibodies reacting with 22- and 18-kDa proteins detected in a standard preparation of antigens was significantly lower in the serum of infertile patients with endometriosis (0.20 +/- 0.05 and 0.57 +/- 0.10) and without endometriosis (0.21 +/- 0.06 and 0.53 +/- 0.08) compared to that of control fertile women without endometriosis (0.53 +/- 0.08 and 1.09 +/- 0.13). After purification by chromatography, the NH2-terminal amino acid sequence of the proteins in the 22- and 18-kDa range was identical through 20 amino acids with the alpha chain of the human haptoglobin. Double immunodiffusion implied immunochemical identity between commercial human haptoglobin and the purified proteins. We conclude that infertile patients with and without endometriosis show reduced serum levels of antibodies against a haptoglobin-like protein. These results would indicate an alteration of the immune system or changes in the levels of these antigens in infertility and/or endometriosis.