The effect of a viral immunomodulator rigvir on humoral immunity was studied in patients with skin malignant melanoma. Peripheral blood levels of B-lymphocytes, IgA, G and M and circulating immune complexes were assayed and immunoglobulin/B-cell ratio (Ig/B) calculated. Preoperative treatment with rigvir brought the indexes of humoral immunity to normal. Response of melanoma patients to rigvir treatment was different from that seen in healthy subjects and was determined by the course of disease.