Acute myeloid leukaemia results from the neoplastic transformation of haematopoietic stem cells. Although advances have been made in its treatment, the mortality rate remains high. As a result, therapeutic alternatives continue to be explored. In this study, we present evidence that suggests that casein, the principal protein in milk, possesses significant antileukaemic properties. We investigated whether casein inhibited the in vitro proliferation and induced the apoptosis of the mouse myelomonocytic leukaemia cell line WEHI-3. By contrast, under identical conditions, casein markedly promotes the proliferation of mouse normal mononuclear bone marrow cells. Since the selective elimination of leukaemia cells is an ideal therapeutic strategy, we also evaluated the antileukaemic potential of casein in vivo. The results showed that casein increases the survival of mice bearing WEHI-3-induced tumours, suggesting that this molecule is also capable of inhibiting the proliferation of these cells in vivo. The evidence that casein inhibited cell proliferation and induced apoptosis in leukaemia cells in vitro, but increased survival in vivo in a leukaemia mouse model, indicates that casein may be useful in leukaemia therapy.