Interferons (IFN) elicit antiviral and antineoplastic activities by binding to specific receptors on the cell surface. The binding characteristics of IFN to human lymphocytes were studied using IFN α2 labeled with 125I to high specific activity. The specific binding curves generated were analyzed by the LIGAND program of Munson and Rodbard to determine receptor numbers. The number of receptors in peripheral blood lymphocytes (PBL) and tonsillar B-lymphocytes (TBL) from normal individuals were 505 ± 293 (n = 10) and 393 ± 147 (n = 3) respectively. When these cells were preincubated in vitro with unlabeled IFN α2, the receptor number decreased to 82 ± 45 and 61 ± 16 respectively. Receptor binding activities recovered gradually over a period of 72 h when the cells were incubated in IFN-free medium. This recovery of receptors could be blocked by the addition of actinomycin D to the incubation medium. A similar decrease in receptor expression was observed in vivo in PBL from patients being treated daily with 5 x 106 units/m2 per d of IFN α2 by subcutaneous injection, for acute lymphoblastic leukemia or papilloma virus infections. Receptor numbers in PBL in vivo were further reduced concurrent with the progression of IFN therapy. Thus the reduction in IFN receptor expression observed in vitro can be demonstrated in vivo. These studies indicate that monitoring IFN receptor expression in vivo can provide information regarding the availability of IFN receptors at the cell surface for the mediation of IFN actions during the course of IFN therapy.