Blood plasma interferon (IFN) of patients with acute myeloid (AML) and chronic granulocytic (CGL) and with acute and chronic lymphoid leukaemia (ALL and CLL) was measured by bioassay and characterized by neutralization with anti-human-IFN-alpha treatment. Elevated IFN-alpha level (60-125 IU/ml) was found in the quiescent phase of CGL as compared to the control samples (15 +/- 10 IU/ml). No significant differences could be found between plasma IFN of patients suffering from the blastic crisis of CGL and control persons. Analogous results were obtained in experiments with plasmas of AML patients, regardless the stage of the disease. Elevated IFN-alpha production (60-100 IU/ml) was found in patients being in the remission phase of T-cell ALL but only in one case in the progressive phase of the disease. No significant elevation of plasma IFN level was demonstrated in patients with O-cell ALL and B-cell CLL, as compared to the control samples.