Seven patients with type II idiopathic mixed cryoglobulinemia were treated with recombinant human leukocyte interferon (alpha interferon). In all of them, a conspicuous reduction of circulating cryoglobulins was noted, together with a definite, remarkable improvement of the clinical pattern. The immunologic parameters (natural killer cell activity and phagocytosis, among others) improved as well; side effects were usually mild and transient. Increases in the cryoglobulin level occurred in a few cases, but they were at least partly sensitive to readministration of alpha interferon treatment. The favorable results obtained in these cases of idiopathic cryoglobulinemia seem to be consistent and prolonged.