Using an antiserum against carcinoembryonic antigen, which was free from non-specific cross-reacting antigen activity, carcinoembryonic antigen has been demonstrated in 45 out of 90 breast carcinomas by an indirect three-stage immunoperoxidase method. The presence of carcinoembryonic antigen appears to be related to good histological differentiation but not to histological type, lymph node metastasis, or recurrence within two years of primary diagnosis. It is suggested that the varying results obtained by different workers may be due to the differing characteristics of the anti-carcinoembryonic antigen serum used.