An indirect immunofluorescent method has been used to study the distribution of the membrane protein beta 2-microglobulin in normal, benign and malignant human breast tissue. A uniform staining of epithelial cell membranes has been found in all normal and benign tissues, with only a minor variation in the intensity of reaction between samples. In contrast the carcinomas have shown a variety of reactions, with some behaving in a manner similar to the normal and benign breast tissue, others being heterogeneous with regard to intensity of staining and the remainder having a weak or a negative reaction. The immunohistochemical behaviour of the carcinomas has paralleled the degree of histological differentiation, with poorly differentiated carcinomas showing little reactivity. A comparable relationship with lymph node status has not been found. Although those carcinomas with a staining pattern akin to non-malignant tissue have a lower incidence of lymph node metastasis it has been the heterogeneous reacting group rather than the weak staining and negative tumours which has shown a higher rate of axillary lymph node metastasis. The prognostic significance of this finding is at present uncertain and can probably only be assessed from information about patient survival.