Interleukin Iα (IL-1) is a potent agent that induces a wide range of biological effects. In this study we analysed its effects on cell cycle progression and differentiation of Daudi Iymphoma cells. The parallel analysis in light microscopy and cytofluorimetry by means of anti-BrdU monoclonal antibodies showed a reduced rate of proliferation (S phase) with a G1 arrest. These features were confirmed by the lower incorporation of [3H]-thymidine supporting the decrease in the rate of DNA synthesis. In addition this cytokine was able to induce differentiation after 24 hrs of treatment as assessed by the increased expression of Fc receptors (FcR) and morphological criteria. This multiparametric analysis gives evidence to the sensitivity to this cytokine of this peculiar cell line.