Because IL-6 has been involved in the pathogenesis of acute monoblastic leukemia, we investigated the in vitro anti-proliferative effect and the in vivo anti-tumoral effect of an anti-IL-6 murine monoclonal antibody (mAb) in a patient with M5B type acute leukemia. In the current study, we clearly show the IL-6 dependence of monoblastic cell viability and proliferation in vitro in short-term cultures of malignant cells and the clinical activity of the anti-IL-6 murine mAb. The complete neutralization of IL-6 in vivo was associated with a transient but complete disappearance of malignant monoblastic cells in the peripheral blood, with improvement or even normalization of several other biological parameters of disease activity. No immunization against the anti-IL-6 murine mAb was observed.