The immunoreactive insulin contents of the cytosol extracts of 57 human mammary carcinomas were measured by specific radioimmunoassay. All of these extracts contained measurable insulin (detection limit 1.3 ng/g tumor). In some cases (23) the levels were quite greater than the normal human serum levels. Immunoperoxidase technique revealed the presence of immunoreactive insulin within the cytoplasm of tumor cells in 6 cases.