With the method used, the concentrations of carcinoembryonic in antigen serum can be said to be abnormal above 10 ng/ml. The concentrations are normal in benign mastopathies. In 131 cases of breast cancer were studied, they are high in 8 p. cent of T1 and T2 breast cancers, and in 53.9 p. cent of T3 and T4 cancers. They are high in 22 p. cent of cases in the first perceptible phase of their disease, and 92 p. cent relapsed cases. They are also high in 23 p. cent of cases without metastasis, in 32 p. cent of cases with only nodal metastasis, in 82 p. cent of cases with extra-nodal metastasis, and in 94 p. cent of cases with nodal and extra-nodal metastasis. Concentrations of carcinoembryonic in antigen serum which are abnormal before treatment and which do not completely return to normal after treatment, of which rise again, enable one to predict the appearance of a metastasis several months before its clinical manifestation. Determination of the concentrations of carcinoembryonic in antigen serum therefore deserves to become part of the battery of routine for all breast cancers. An initial high concentration is an argument for suggesting systematic chemotherapy. The fluctuations in the concentrations of carcinoembryonic in antigen serum under chemotherapy allow one to assess the efficiency of the treatment.