A preliminary survey of carcinoembryonic antigen (CEA) concentrations in 23 breast-cyst fluids (BCF) and blood plasma has revealed markedly increased concentrations of CEA-like material in BCF as compared to its concentrations in plasma. Chromatography on Sephadex G-200 indicated the immunoreactive CEA component extracted from BCF corresponds to a glycoprotein with a molecular weight of at least 200 000.