Patients receiving large amounts of interferon (IFN), particularly cloned IFN-alpha, have been reported to develop antibodies to IFN-alpha. We used a neutralization technique bioassay to monitor 175 patients undergoing recombinant (r) IFN-alpha 2b therapy in an attempt to correlate antibody production to disease progression. Only 1 patient being treated for chronic myelogenic leukemia produced antibodies. Our findings indicate that rIFN-alpha 2b has minimal antigenicity, even when administered for prolonged periods of time.