Recombinant interferon alfa, a natural product with growth inhibitory capabilities, has been demonstrated for the first time to have significant therapeutic efficacy in controlling the red cell mass in patients with the myeloproliferative disease polycythemia vera. The starting dose was 3.0 X 10(6) U three times a week, subcutaneously (SC). In three patients the dose required was 5.0 X 10(6), U five times a week, SC. Side effects were easily tolerated. The striking advantage in the use of this drug is its presumed absence of antileukemic effect. Further evaluation is necessary, but the initial responses are encouraging.