In 1981, the National Cancer Institute undertook Phase II trials of interferon alfa-2a in patients with non-Hodgkin's lymphoma (including cutaneous T-cell lymphoma [CTCL]) and chronic lymphocytic leukemia (CLL). A dose of 50 X 10(6) U/m2, three times per week, was used initially, then adjusted downward as dictated by toxic effects. A 54% response rate was achieved among 24 patients with low-grade non-Hodgkin's lymphomas, and the median duration of response was 8 months. Less encouraging results emerged from studies in patients with intermediate- or high-grade disease. Responses were noted in only two of six patients in the former group, and only one of seven in the latter group. Results have likewise been disappointing in patients with CLL. Of 18 individuals treated, only two exhibited brief, partial responses. In CTCL, on the other hand, alpha interferon may be the most effective single agent. Among 20 patients with advanced disease who had failed previous therapies, 45% responded. The primary dose-limiting toxicity in all these trials has been flu-like symptoms, particularly fever and fatigue. Fever has generally resolved as treatment has been continued, but dosage reductions are usually necessary to alleviate fatigue. Future studies are likely to focus on the use of alpha interferon in combination with chemotherapeutic agents or other biologic response modifiers, such as monoclonal antibodies.