Abstract: To study a automatic detecton of micronucleus in Chinese hamster ovary (CHO) cells in vitro by laser scanning cytometry(LSC),the micronucleus of CHO cells induced by cyclophosphamide (CP) were detected by LSC conbined with cytokinesis block micronucleus assay using two kinds of fluorescent dyes of fluorescein isothiocyanate (FITC,a protein-specific dye) and propidium iodide (PI,a DNA-specific dye).Also,the micronucleus frequencies of CHO detected by LSC were compared with that calculated by a microscope here.Binucleated cells and micronucleus in the binucleated cells can be identified by LSC accurately;and the micronucleus frequencies of binucleated cell induced by CP in the cytokinesis block micronucleus assay were found to be significantly higher than the control.Besides,the frequencies of micronucleus in CHO cells were increased along with higher concentration of cyclophosphamide.And the results of in vitro micronucleus scored by LSC have a good correlation with that by the traditional microscopy.LSC is an easily operated method,and it detects micronucleus in CHO cells rapidly and accurately,as well as economically.It is suitable for detecting micronucleus of in vitro cells,especially for attachment cells.