The effect of three alkylation agents MNNG, ENNG and DMS on HeLa cells grown in culture were examined and the pattern of DNA distribution in cells were analysed by flow cytofluorometry. The results show the cell division was partly inhibited and the rate of DNA synthesis in cells decreased after treatment of MNNG. The proportion of cells in S phase increased with increasing of exposure time to above control levels. DMS shown a similar phenomenon but ENNG only caused a little increase of S phase cells.