Twenty-seven patients with Philadelphia chromosome positive chronic myelogenous leukaemia in the chronic phase were treated with low doses of recombinant interferon (IFN) alpha-2b. Ten patients entered a complete and six a partial haematologic remission with a median duration of 5.8 and 9.1 months respectively. Five minor cytogenetic responses were observed. These results are inferior compared to other studies with higher interferon-doses. Fever was an acute side effect after injection of IFN, limb pains and fatigue occurred protractedly. Haematologic side effects, nonspecific EEG changes, weight loss, and development of pulmonary infiltrates were observed in later periods of the treatment. Eight patients developed neutralizing anti-IFN antibodies after 4.2-20.4 months (median 12.8 months). Anti-IFN antibodies were associated with relapse or refractoriness to IFN treatment: five out of nine patients with rising WBC after initial fall had antibodies, while four did not. Two out of four patients with primary non-response had IFN-antibodies. These results may indicate a serious problem in the long-term treatment of CML with recombinant interferon.