Abstract: A method of measuring chain breakage in the DNA of a eucaryotic cell is disclosed. This method includes (a) contacting the cell with a stripping solution that lyses and solubilizes the cell without denaturing its DNA, thereby forming a nucleoid having a halo, (b) measuring the width of the halo, and (c) determining the number of chain breaks from the measured width. The halo includes at least one loop of undenatured DNA, and has a width related to the number of DNA chain breaks in the loop. The cell to be examined may be adhered to a support prior to its contact with the stripping solution. The number of DNA chain breaks can be determined by compairing the measured width of the nucleoid halo with a reference value.