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 microM) 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 microM, 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 0.5-1.5 microM 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.