(1) Homogenates of rat mammarytumours induced by 7,12-Dimethylbenz(a)anthracene cause the formation of specific antisera in rabbits. (2) Absorption against relevant normal tissues and rat serum eliminates non-specific precipitations. (3) By the Ouchterlony double-diffusion method in agar, all the tumours tested formed well-defined precipitation lines against the antisera from immunized rabbits, within 24 h at room temperature. (4) Precipitation lines were observed regardless of the histological type of tumour from which the homogenate was obtained. The tumour histology ranged from examples of malignant adenocarcinomas to benign hyperplasia. (5) The process of growth, be it benign or malignant at the time when the tumour was removed seems to be closely associated with the presence of a specific protein, which by the method used could not be detected in simple mammary hyperplasia (due to pregnancy or lactation).