The influence of interferon-alpha (IFN-alpha) on the differentiation of malignant cells from six patients with chronic lymphocytic leukaemia was studied in vitro. IFN induced differentiation in the leukaemic cells from four of the patients. In cells from two of these patients, IFN also induced proliferation. When tested by immunofluorescence, the clonality of the differentiating cells was established by the presence of intracellular light chains of one type only.