The incidence of therapy-induced neutralizing interferon-beta (IFN-beta) antibodies was investigated in 41 Japanese patients with melanoma treated with a combination of chemotherapy and subcutaneous injections of natural IFN-beta. Only one patient (2.4%) developed anti-IFN-beta antibodies. This finding differs from those of previous studies with Caucasian patients in which subcutaneous injections of natural IFN-beta resulted in a high incidence of anti-IFN-beta antibodies. This difference may be related to ethnic group of patients, treatment schedule, pharmacological preparation of IFN-beta and/or technical assay system used in each study.