The pesticides, chlorpyrifos, Alpha-thrin, Efekto virikop and springbok were assessed for cytotoxicity and genotoxicity in the onion root tip assay. Onion seeds were germinated on moistened filter paper in petri dish at room temperature until radicles appeared. Germinated seeds were exposed to three concentrations of each pesticide for 20 h. About 1 – 2 mm length of root tip was cut, fixed in acetic alcohol, washed in ice cold water, hydrolyzed in warm 1 N HCl, stained with aceto-carmine and squashed on glass slide. For each treatment, about 3000 cells were scored and classified into interphase and normal or aberrant division stage. Cytotoxicity was determined bycomparing the mitotic index (MI) of treated cells with that of the negative control. The MI of cells treated with chlorpyrifos, Alpha-thrin or springbok was half or less, that of the control at one or more doses and adjudged cytotoxic. Efekto virikop was not cytotoxic. Genotoxicity was measured by comparing the number of cells/1000 in aberrant division stages at each dose with the negative control using the Mann-Whitney test. Chlorpyrifos was genotoxic (P < 0.05), inducing chromosome lagging and bridges, pulverized and stick chromosomes, multipolar anaphase and telophase. Efekto virikop and springbok induced lagging chromosomes. Alpha-thrin was not genotoxic.

Key words: Allium cepa, cytotoxicity, genotoxicity, mitotic index, pesticides, root tip cells.