Nimbolide, a natural triterpenoid present in the edible parts of the neem tree ( Azadirachta indica), was found to be growth-inhibitory in human colon carcinoma HT-29 cells. Nimbolide treatment of cells at 2.5 - 10 microM resulted in moderate to very strong growth inhibition. Flow cytometric analysis of HT-29 cells showed that nimbolide treatment (2.5 microM, 12 h) caused a 6.5-fold increase in the number of cells (55.6 %) in the G2/M phase compared with the control cells (8.8 %). At 48 h, the cell population in the G2/M phase decreased to 18 %, while that in the G0/G1 phase increased to 52.3 %. Western blot analysis revealed that nimbolide-mediated G2/M arrest was accompanied by the up-regulation of p21, cyclin D2, Chk2; and down-regulation of cyclin A, cyclin E, Cdk2, Rad17. At G0/G1 cell cycle arrest, modulation in the expression of the cell cycle regulatory molecules was also observed. We found that nimbolide-induced growth inhibition and cell cycle arrest were not associated with cellular differentiation. Quantification of cells with respect to the expression of phosphatidylserine in the outer cell membrane showed an increase in apoptotic cells by about 13 % after 48 h of nimbolide treatment.