An antigen present in human breast tumor cells, and which is immunologically related to the envelope protein (gp52) of murine mammary tumor virus, was used as a marker for the detection of breast cancer in an Israeli population. The results show that the antigen was detectable in 128 of 204 breast carcinomas tested (62.7%). The immunological reaction was not detected in normal breast tissue, benign breast tumors, ductal hyperplasia or in primary malignancies in other organs. A significantly higher percentage of cases with demonstrable antigen was found in Israeli women born in North Africa (78%) as compared to women of European origin (60.6%). The frequency of detection of the antigen was higher in stage IV (80%) as compared to stage I (15%), suggesting that the gp52 cross-reacting antigen is a marker for the severity of the disease. Moreover, a retrospective study of 97 cases of stage II breast cancer shows that if the antigen is detected at the time of mastectomy, one can usually predict an unfavorable prognosis. Survival data analysis indicates that patients without detectable antigen survived significantly longer than those with a detectable antigen.