In vitro data suggest a synergistic antiproliferative effect of different cytokines. In four clinical studies chronic myelogenous leukemia (CML) patients were treated with interferon (IFN)-alpha alone or IFN-alpha combined with either low-dose IFN-gamma or tumor necrosis factor (TNF)-alpha. The best response was achieved in previously untreated patients with good prognostic factors and highest tolerable IFN dose for maintenance treatment. Breakpoint localization within the major breakpoint cluster region did not correlate with response to IFN. In a randomized study of IFN-alpha versus IFN-alpha combined with IFN-gamma, no differences in response rates were observed. Patients with primary or secondary resistance to these treatment modalities received a combination therapy with IFN-alpha and TNF-alpha. In these patients, a decrease in leukocyte counts was noted, but no cytogenetic improvement occurred.