Alpha-interferon (alpha-IFN) and gamma-interferon (gamma-IFN) are highly effective biologic agents in the treatment of chronic myelogenous leukemia (CML). 4-hydroperoxycyclophosphamide (4-HC) is the most commonly used chemotherapeutic drug when purging bone marrow contaminated with malignant cells. To investigate the efficacy of alpha-IFN, gamma-IFN and 4-HC as purging agents in CML patients, K562 cells, a human CML cell line, were treated in vitro with these drugs singly or in combination. The cytotoxic effect was evaluated by MTT assay. Data were analyzed for synergism by the median effect principle of Chou and Talalay. Using Southern blot analysis, the effect of purging on the suppression of bcr gene rearrangement was also evaluated. Alpha-IFN, gamma-IFN and 4-HC all showed good concentration-dependent cytotoxic effect. Median cytotoxic doses of alpha-IFN gamma-IFN and 4-HC were 0.03 microM 0.20 microM and 1.97 microM respectively, and molar ratios of median doses of alpha-IFN to 4-HC, gamma-IFN to 4-HC and alpha-IFN to gamma-IFN were 1:80, 1:10 and 1:8, respectively. The combination treatment of alpha-IFN and 4-HC, and gamma-IFN and 4-HC showed significant synergistic cytotoxic activities, but combination of alpha-IFN and gamma-IFN was antagonistic. Status of bcr gene rearrangements in K562 cells was not changed after the treatments. Our results appear to provide an efficient method for removing CML cells from the bone marrow of CML patients.