Enhanced garlic (Allium sativum) consumption is closely related to reduced cancer incidence, as shown in epidemiological studies. Diallyl disulfide (DADS), a component of garlic, inhibits the proliferation of human blood, colon, lung and skin cancer cells. Although DADS had been reported to induce apoptosis in human leukemia HL-60 cells, there are no reports regarding whether or not it affects leukemia cells in vivo. Therefore, the present study is focused on the in vivo effects of DADS on WEHI-3 leukemia cells. The effects of DADS on murine WEHI-3 cells were initially examined, and the results indicated that DADS induced cytotoxicity and that this effect was dose-dependent. The effects of DADS on WEHI-3 in BALBIc mice were also examined, and the results indicated that DADS decreased the percentage of MAC-3 marker, indicating that differentiation of the precursor of macrophage and T cells was inhibited. The weights of liver and spleen were also measured, and the results indicated that DADS decreased the weight of these organs. An important characteristic of WEHI-3 leukemia is the enlarged spleen in mice after i.p. injection of WEHI-3 cells. Based on pathological examination, the function of DADS was observed in the liver and spleen of mice previously injected with WEHI-3 cells. Apparently, DADS affects WEHI-3 cells both in vitro and in vivo.