In an ongoing phase-II trial we aimed to predict clinical responsiveness of Philadelphia chromosome positive (Ph1+) chronic myelogenous leukaemia (CML) to recombinant IFN-alpha-2C (rIFN-alpha-2C) by pretesting in vitro. From five normal controls and 14 CML patients in chronic phase, bone marrow samples were taken before treatment and tested for antiproliferative activity by rIFN-alpha-2C, using a microagar culture system for BFU-E, CFU-GM, and CFU-Meg. Light-density nucleated bone marrow cells were stimulated for BFU-E and CFU-Meg colony formation with Alpha medium containing 20% serum obtained from a patient with severe aplastic anaemia. CFU-GM growth was induced with conditioned medium from the cell line GCT. In normal controls BFU-E, CFU-GM and CFU-Meg colony formation was inhibited by rIFN-alpha-2C in a dose-dependent manner. BFU-E proved to be the most sensitive cell lineage (IC50: 65; range: 53-116 U/ml) whereas CFU-GM was about 20 times less sensitive (IC50: 643; range: 480-897 U/ml). The sensitivity of CFU-Meg ranged between these two colony types with 50% growth inhibition at an IFN concentration of 160 (range: 68-246 U/ml). A heterogeneous response to rIFN-alpha-2C in vitro was seen in CML patients. Three of the 14 patients were 'resistant' to rIFN-alpha-2C in vitro with IC50 values for BFU-E, CFU-GM and/or CFU-Meg colony formation greater than 10(4) U/ml. Patients were subsequently treated with a daily dose of rIFN-alpha-2C of 5 x 10(6) U. Four patients achieved a complete and six achieved a partial haematological response. Of the four non-responders three rapidly progressed into blastic crisis. Thus it was seen that treatment failure to interferon was accompanied by IFN-resistance in vitro of BFU-E, CFU-GM and/or CFU-Meg colony formation by bone marrow precursors (p less than 0.01). These results suggest a predictive value of IFN-sensitivity testing in vitro in Ph1 + CML.