Treatment with interferon in solid tumors have not generated as high response rates (RR) as in hematological malignancies. So far, an antitumor effect of a-IFN has only been noticed in a limited number of cancers e.g. malignant melanoma (RR∼25%), renal cell cancer (RR∼16%) and neuroendocrine gut and pancreatic tumors (biochemical response in ∼45%, tumor responses in ∼15–20%). Addition of chemotherapeutic drugs have significantly increased the response rates in malignant melanoma (RR∼35%) but not in renal cell cancer or neuroendocrine tumors. A combination of a-IFN plus 5-FU and/or leucovorin has generated significant antitumor effect in colorectal cancer (RR-38%).