A transferrin receptor was demonstrated in tumor tissue from 10 patients with breast carcinoma and one patient with breast sarcoma. Binding studies were conducted by measuring the amount of 125I-transferrin binding to microsomal preparations of the tumor tissue. Elevated levels of specific transferrin binding were found in the tumors with a range of 11–35% of bound transferrin, whereas microsomes prepared from non-neoplastic breast tissue samples bound only 2.3% and 2.4% of the transferrin. Scatchard analysis of binding studies conducted with tissues from a breast cancer and from a breast sarcoma indicate that the receptor has a Ka = 9.0 × 108M. The binding site is specific for transferrin, as studies show that non-radioactive transferrin displaced labelled transferrin, while human IgG and human albumin did not. The receptor-transferrin complex was precipitated from a detergent extract of the breast sarcoma with antiserum to human transferrin. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the immunoprecipitate gave a polypeptide of Mr 90,000 daltons, which is of similar molecular weight found for the putative transferrin receptor in all of a series of human cultured cell lines previously examined.