Approximately 3-20% of all reticulocytes in blood of healthy persons are immature and transferrin receptor positive (Tf-Ret). Tf-Ret were measured by flow cytometry in 27 patients treated with three different radiopharmaceuticals labeled with (131)I and in 25 healthy controls. Patients were chronically exposed within 6 days to blood doses from 0.18-1.89 Gy (D6). Typically, two-thirds of D6 was administered within the first day (D1). The study had to be confined to intra-subject investigations due to high biological variability of Tf-Ret counts. A significant radiation-induced decline was found in patients D1 doses that were ≥0.5 Gy. Tf-Ret frequency declined during the first 4 to 5 days of nuclear therapy to about 30-60% of its initial value, and increased in the following 3 days without reaching the initial value. At the time of nadir, the relative frequency of Tf-Ret was more depressed than that of reticulocytes and lymphocytes. The relative Tf-Ret frequency at nadir could be fitted to the equation: %-Tf-Ret=exp-(D1/D(o)). D(o) was found to be 1.0 ± 0.4 Gy (Mean ± SEM). The study shows that Tf-Ret frequency in blood might be a good parameter for estimation of the radiation dose to red marrow.