The influence of 60Co gamma-radiation on the sedimentation behaviour of human lymphocyte nucleoids in neutral sucrose gradients was studied with the aid of a modified fluorescent monitor system consisting of the DNA-binding dye, Hoechst 33258 and a vertical centrifuge rotor. 2. A combined method employing the sedimentation procedure and the incorporation of tritiated nucleosides was utilized to evaluate the repair of gamma-radiated DNA.