This study examines the cytological effects of coumarin (2H-1-benzopyran-2-one) on Lens culinaris Medik. cv. Sultan in terms of cell response, mitotic index, mitotic abnormalities and chromosome aberrations. Effective concentration values were calculated based on a probit model, which is a type of regression where the dependent variable can take only two values, i.e., life or death, after 72 hours of treatment. The effective concentration value of EC50 was determined via probit analyses, approximately 278 mu M, and was then adjusted to 300 mu M. The bulb roots were treated with the following concentrations depending on the root growth inhibition test: 300 mu M (EC50), 600 mu M (2X EC50). The control group was treated with Hoagland. Germination percentages of L.culinaris roots decreased with increasing coumarin concentrations. Cytological observations demonstrated that the mitotic frequency in root meristematic cells lessened and that abnormality frequency decrease parallel to the increase in concentrations for coumarin. The obtained results indicated that this active ingredient had the ability to cause a reduction in the seed germination percentage in the various phases of mitosis and increased chromosomal aberrations in L.culinaris meristematic cells.