Gnidilatimonoein is a new diterpene ester, recently isolated from the leaves of Daphne macronata with potent anti-tumoral and anti-metastastic activities (Yazdanparast et al., 2004). Promyeloblastic (KG1), promyelocytic (NB4) and promonocytic (U937) cells were cultured in the presence of various concentrations of the drug (0.5-3.0 ${mu}$M) for 3 days. Herein, we report that gnidilatimonoein induces differentiation and apoptosis in KG1 , NB4 and U937 cells. The drug inhibited growth and proliferation of KG1 , NB4 and U937 cells with IC50 values of 1.5, 1.5 and 1.0 ${mu}$M, respectively, after 72 h of treatment. Cell viability was also decreased by 18%, 20% and 23%, respectively, after 72 h treatment with the drug. NBT reducing assay revealed that the inhibition of proliferation is associated with differentiation especially toward monocytes-like morphology. Indeed, the drug at 0.5-15 ${mu}$M induced differentiation by 5-50% in the cells. Acridine orange/ethidium bromide (AO/EtBr) double staining and DNA fragmentation assays revealed that apoptosis occurred after differentiation of the cells. Based on the present data, it seems that the new compound is a good candidate for further evaluation as an effective chemotherapeutic agent acting through induction of differentiation and apoptosis.