We have previously demonstrated that specific antigens involved in autoimmunity in endometriosis may be endometrial proteins with molecular weight (mw) of 71, 92, and 103 kilodalton (kDa). The purposes of this study were to determine the incidence of IgG antibodies against these endometrial antigens in peritoneal fluid of patients with endometriosis and to evaluate the antigenic differences between the endometria of patients with and without endometriosis. Forty peritoneal fluid (PF) from 24 patients with endometriosis and 16 patients without endometriosis (control patients) were tested against endometrial protein from patients (n=8) with endometriosis and from control patients (n=10) by western blot. Fifteen (62.5%) of 24 PF samples from patients with endometriosis had specific Immunoglobuiin (Ig) G antibodies against one of three endometrial proteins with mw of 71, 92 and 103 kDa but none of PF samples from control patients had these antibodies. The electrophoretic pattern of endometrial proteins from patients with endometriosis was similiar to that from control patients. Furthemore there was no significant difference in specific PF Immunoglobulin G binding to endometrial proteins regardless of origin of these proteins. Our data indicate that specific humoral immune response can be found in PF of patients with endometriosis and that specific antigens inducing this immune response are present in human endometrium and that there is no antigenic difference between the endometria of patients with and without endometriosis.