Plant derived agents may exert a new approach to the treatment of leukaemia. The present study was an evaluation of proliferation, cytotoxicity and differentiation of harmine and harmaline on HL60 cells, alone or in combination with ATRA and G-CSF. Counting of cells, viability, MTT assay, morphology, NBT reduction and flow cytometry analysis were performed using CD11b and CD 14 monoclonal antibodies. The data showed that harmine and harmaline reduced proliferation in dose and time dependent manner. Optimal antiproliferative concentration of these agents was chosen. However, both agents in higher doses were cytotoxic. Combination of ATRA, G-CSF and each agent alone, particularly harmaline in optimal dose, resulted in partially additive decrease in cell proliferation. Cells treated with both harmaline and ATRA demonstrated some morphological changes and NBT positivity, but the extent of changes observed following treatment with harmaline was less than ATRA. Flow cytometric analysis showed that ATRA induced a neutrophilic differentiation, while harmaline led to a predominantly monocytic differentiation. Combination of harmine and harmaline with ATRA and G-CSF did ret change the extent of differentiation, and the cells differentiated into the neutrophilic lineage. This shows that the direction of differentiation is dominantly determined by ATRA. These preliminary data implies a new approach in treatment of leukemia.