Glucocorticoid receptors have been detected in 90 human breast tumors and tumor-like conditions by the immunoperoxidase method using a specific antibody against the glucocorticoid receptor isolated from rat thymocytes. In some of the specimens the [3H]-dexamethasone binding assay was also performed and the results obtained were compared with those of the immunoperoxidase method. When the biochemical method was used, no strict correlation between the degree of binding of [3H]-dexamethasone and malignancy on the basis of histological findings could be demonstrated. In contrast, the immunoperoxidase method was in full agreement with the histological type of the tumor. Thus, nearly all malignant breast tumors (carcinomas) were positive by the immunoperoxidase method to a varying degree. Most of the examined benign tumor-like conditions (fibrocystic disease) were found to be negative. Intermediate situtations, such as atypical duct or lobular hyperplasia, papillomatosis etc., were mostly positive. These findings suggest that the immunoperoxidase method, apart from its usefulness for the detection of glucocorticoid receptors in breast tissue, may be used as an early biological marker to detect early conversion of normal to hyperplastic tissue and/or malignancy of the mammary gland.