Interferons (IFNs) initiate their effects by interacting with specific high-affinity cell surface receptors, but little is known about the physiology of IFN receptor interaction in vivo. Treatment of patients suffering from hairy cell leukemia (HCL) with human recombinant alpha IFN results in significant tumor regression, with clinical improvement in a high percentage of cases. To investigate a possible relevance of binding parameters as response markers, IFN receptor interaction on tumor cells responsive to IFN in vivo was studied. Binding of human alpha 2 IFN to circulating hairy cells was analyzed before and during IFN therapy in ten patients selected on the basis of high numbers of peripheral hairy cells. Binding experiments were carried out on Ficoll-Paque fractionated peripheral cell samples containing a majority of hairy cells. All patients reacted to recombinant alpha IFN treatment with a striking decrease in binding capacity within 12 hours after the first injection. As demonstrated by using a monoclonal antibody able to recognize alpha 2 IFN bound to its receptor, this decreased binding capacity was not due to blocking by circulating IFN but rather to a decrease in receptor number. This receptor "down-regulation" was partially reversible after the first IFN injection. However, upon prolonged IFN therapy, all patients displayed a stable state of decreased receptor expression. Down-regulation of IFN receptors can be regarded as a response marker to IFN treatment. This response marker, however, was not correlated with the clinical response within the first months of IFN therapy.