Low-grade non-Hodgkin's lymphoma and multiple myeloma are chemosensitive malignancies, but are rarely curable because of primary or acquired drug resistance. Interferon has been shown to modulate the multidrug resistance phenotype and to reinduce chemosensitivity in patients with chemoresistant tumors. Fifteen patients with multiple myeloma and 64 patients with low/intermediate grade non-Hodgkin's lymphoma unresponsive to initial chemotherapy were treated with alpha 2b interferon for 2 months. In case of an objective response, treatment was continued until disease progression; non-responding patients received the same chemotherapy to which they were resistant, preceded by a 5 day course of interferon. Interferon salvage monotherapy induced an objective response in 1/15 patients with multiple myeloma and in 7/64 patients with non-Hodgkin's lymphoma. An objective response was achieved after retreatment with first-line chemotherapy preceded by interferon in 4/14 patients (28.6%) with multiple myeloma and in 20/56 evaluable patients (35.7%) with non-Hodgkin's lymphoma. Toxicity was moderate, predictable, manageable, and never caused interruption of the treatment. Interferon appears to be able to modulate chemosensitivity of tumors refractory to chemotherapy with several potential mechanisms, including an effect on drug accumulation; its utilization in this setting warrants further evaluation.