Neutralizing anti-IL-2, anti-IL-3, and anti-IL-6 monoclonal antibodies (MAbs) were used for the immunoenzymatic detection of the respective cytokines in blast cells of 38 patients with acute myeloid (AML) and lymphoid (ALL) leukemias by the APAAP-technique. In 20/24 AML-cases (83%) blast cells showed intranuclear staining with MAb anti-IL-2 (DMS-1). In 17 cases reaction was restricted to the nucleoli, in 4 cases additional cytoplasmic staining was observed. Only 2/13 (15%) of the ALL cases showed anti-IL-2 staining. In contrast to IL-2, neither IL-3, IL-6 nor IL-2R alpha-chains were detected in any of the acute leukemias tested. The anti-IL-2 staining of nucleoli in AML cells is distinct from the cytoplasmic staining which is observed in PHA-activated normal lymphocytes and in a minority of AML cells.