The response to oophorectomy in 40 patients with advanced breast cancer was evaluated as a function of the presence or absence of the antigens recognized by the monoclonal antibodies MBrl, MBr8, and MOv2 on the cell surface of the primary tumor. Two groups (20 patients alive 5 years after oophorectomy and 20 patients who died within 5 years) were evaluated. These groups had a) the same distribution of lymph node status, diameter of the primary, and the presence of estrogen receptors at the time of first treatment and b) a comparable extent of the disease at the time of oophorectomy. The frequency of complete response after oophorectomy was significantly higher in patients whose tumors did not express the antigens recognized by antibodies MBrl and MBr8. Fourteen of the 22 patients with tumors not reacting with MBrl and 11 of the 18 negative to MBr8 had a complete response. There was no correlation with response and the monoclonal antibody MOv2. Both MBrl and MBr8 were present in 11 patients and both were absent in another 11. The simultaneous absence of the two antigens was related to the highest complete response rate (9 out of 11 patients).