AbstractIn an attempt to differentiate between the biochemical characteristics of hormonal-dependent and independent gynecologic tumors, monoclonal antibodies (mABs) MBrl and MBr3 were used to detect tumor antigens. Steroid receptors and MBrl and MBr3 determinants were assayed independently in a double-blinded manner. Of the 45 gynecologic tumors analyzed, 16 of 20 (80%) with both progesterone receptors (PR) (>10 fm/mg protein) and estrogen receptors (ER) (>5 fm/mg protein) also contained high levels of MBrl determinant. However, these receptors were not quantitatively related to MBrl activity. The 15 of 17 (88%) tumors that contained receptors below the discriminant levels also had low or undetectable levels of MBrl activity. In tumors where only either PR or ER was present, 2 of 2 PR+/ER−tumors and 2 of 6PR−/ER+tumors expressed MBrl antigen. No relationship of MBr3 antigenic activity to the receptors was established. These results show that MBrl-recognized antigen is expressed largely in steroid receptor-positive tumors and these data suggest that either the antigen itself or its carrier protein may be regulated by estrogen. Like steroid receptors, MBrl antigens may have important prognostic value in gynecologic tumors.