Serum transferrin receptor (s-TR) levels in acute leukemia patients were measured by a recently developed sandwich radioimmunoassay. The mean s-TR level for normal subjects (n = 205) was 246 +/- 79 (mean +/- 1 SD) ng/ml. The values for patients with untreated acute myelocytic leukemia (AML, n = 18) and untreated acute lymphocytic leukemia (ALL, n = 14) were 398 +/- 175 ng/ml and 479 +/- 176 ng/ml, respectively, both of which were significantly higher than those for the normal subjects (AML, p < 0.02; ALL, p < 0.05). When complete remission was achieved with initial remission induction therapy, s-TR decreased to 262 +/- 47 ng/ml (n = 22, 12 AML and 10 ALL), returning to normal levels. There was a good correlation between s-TR levels and the number of leukemic cells in peripheral blood (r = 0.743, n = 32, p < 0.01). In two patients with AML, serial changes of s-TR values and numbers of blast cells in the peripheral blood and bone marrow occurred in a parallel manner. If, therefore, this assay for s-TR can be made more sensitive, it may become useful for assessing acute leukemia activity and for monitoring the effects of therapy.