It was found that both interferon-beta and interferon-alpha had a direct cytotoxic action against a few sensitive cultured cell lines. The action was not concentration dependent, but time dependent. Interferons expressed their maximum effect when they were given daily for long time to nude mice bearing transplantable ascitic human cancer cell lines, indicating that interferons had schedule dependency. In preliminary phase II study of interferons (3-9 X 10(6) U daily im, iv or local use) to 40 patients with various malignancies, only one patient with B-cell lymphoma responded to systemic use of both interferons and two out of five patients with pleural malignant effusion of breast cancer also responded to intrapleural use of interferons. Side effects were usually mild, but included fever, weakness, gastrointestinal, hematopoietic, liver and kidney function disturbances.