50 patients with primary breast cancer were studied to determine the CEA and HCG contents in their tumor cells before any treatment was initiated. Tumor cells were obtained by needle biopsy and each tumor cell population was stained by immunofluorescence. In 21 of the 50 patients, CEA containing cells were found in a frequency ranging from 5 to 80% of the tumor cell population. The results were confirmed by radioimmunoassay of tumor extracts. No apparent relation was seen to cytologic type or grade of differentiation. HCG was detected by IF in 4 tumors with an apocrine cytologic cell type. The level of CEA in plasma was determined before treatment and followed for 2-6 months in 72 patients. CEA was the only measured serum parameter that correlated initially with size and extent of the localized tumor. It was too low to be of use for monitoring primary disease, but should be of value in early detection of metastasis. Posttreatment a low or decreased plasma CEA was seen more often in patients who had had curative treatment than in those given palliative radiation. No raised serum HCG levels were found. Raised serum liver enzymes did not predict the extent of the primary tumor but may be an indication of distant spread. Tumor CEA content and CEA plasma concentration were correlated, although not very strongly. This means that CEA, although present in the tumor, is not always released in measurable amounts.