AbstractAssays of binding of125iodine‐labeled (125I) human transferrin were used to study transferrin receptor sites on reticulocytes from 15 normal subjects and from 66 patients with various hematologic disorders. In normal subjects, few or no transferrin receptors were detected whereas the average number of receptors per reticulocyte varied greatly from patient to patient, ranging from 0 to 67,700 in samples, from 35 patients, on which Scatchard analysis of binding of [125I]‐transferrin was done. Marked heterogeneity in the number of reticulocyte transferrin receptors in different hematologic disorders was also found in assays with [125I]‐OKT9 (monoclonal antibody to the human transferrin receptor). The number of receptors was not correlated with either the reticulocyte count or the hemog