In vitro and in vivo studies utilizing a combination of leukocyte interferon-alpha (IFN) and chlorambucil (CLB) were done to investigate possible synergism between a biological response modifier and a chemotherapy drug. In vitro studies utilized a human myeloid leukemia cell line (K-562) pretreated with IFN and then exposed to CLB. The combination resulted in significant depression of cell growth compared with use of IFN or CLB alone. In vivo studies involved eight heavily pretreated patients given 6 million units IFN for 5 days followed by oral CLB (16 mg/m2) for 5 days repeated every 4 weeks. Three myeloma patients had reduction in immunoglobulins and experienced clinical responses. Three of four patients with Hodgkin's disease responded after relatively short periods of treatment. One patient with a diffuse lymphocytic lymphoma had a complete unmaintained remission lasting 6 months. Toxicity was minimal, with mild fever, nausea, and vomiting. These preliminary studies suggest that IFN may be a biological response modifier when used in combination with a cytotoxic agent.