The clinical significance of radioreceptor assay for transferrin receptors of human cancerous tissues was evaluated. Fresh surgical specimens from various carcinoma tissues were solubilized with 1% Triton X-100 and the extracts were mixed with 125I-labelled diferric transferrin. The free transferrin and the receptor-bound transferrin were separated by 15% polyethylene glycol precipitation. The % specific transferrin binding to gastric, colonic, lung and mammary carcinoma tissues ranged between 3.9 and 13.9%, whereas those for normal stomach and colon were less than 2%. The concentrations of transferrin receptors in these cancerous tissues ranged between 3.7 and 28.3 pmole/g tissues. It was concluded that the amounts of transferrin receptors were significantly increased in all of the tumor tissue extracts examined and may thereby provide a useful marker for the diagnosis of malignancies.