Endometrial antigens from patients with endometriosis and fertile controls were tested against immunoglobulin (Ig)G, IgA, or IgM in endometrium, serum, and peritoneal fluid (PF) of the patients and controls by Western blot analysis. Endogenous IgG was detected in 78% of the endometria or endometriosis implants from the patients and 22% of the endometria from the controls. Endometrial IgA and IgM were detected in few controls and patients. Immunoglobulin G in the serum and/or PF of patients with endometriosis was specifically directed against antigens with molecular weights of 34, 42, 82, 94, 110, 120, and 140 kd found in the patients' endometrium or endometriosis implants. Immunoglobulin A and IgM in the serum or PF of the patients and controls were nonspecific in their reactivity. Endometrial antigens found in endometrium or implants of patients with endometriosis, and eliciting IgG responses, may be relevant to autoimmunity in endometriosis.