To the editor: Interferons have been shown to cause regression in several malignancies (1-3). Although the proper role of interferons in the treatment of cancer has yet to be defined, they are generally regarded as relatively nontoxic. We report here two cases of apparent immune-mediated erythrocyte destruction occurring during interferon treatment. Patient 1: A 56-year-old white man with metastatic renal cell carcinoma was treated with vinblastine, 5 mg/m2 body surface area on day 1 once monthly, combined with alpha-2-interferon, 3 million U/m2 on day 1, 5 million U/m2 on day 2, and 10 million U/m2 on days 3 through 5,