This study will examine the safety and effectiveness of interferon-a and octreotide for the treatment of Zollinger-Ellison syndrome (gastrinoma) and advanced non-B islet cell cancer. Gastrinoma is a tumor produced by the pancreas that secretes the hormone gastrin, which in turn stimulates production of gastric juices that cause ulcers. Some of these tumors are malignant. Gastrinomas that have spread and cannot be surgically removed require drug treatment (chemotherapy). Current drug regimens, however, provide only temporary benefit and, in some cases, produce life-threatening side effects. In studies of patients with tumors similar to gastrinoma, the drugs octreotide and interferon-a, alone or in combination, showed some effect in stopping tumor growth and were better tolerated than chemotherapy. At least one-third of patients responded to treatment with either drug for at least 6 months; the two drugs given together may produce a better response than either one alone.