Benzon nuclease, an endonuclease originating from Serratia marcescens, was tested for its chromosome breaking activity in Chinese hamster ovary cells. Using a permeabilizing method with hypertonic glycerol, benzon nuclease induced chromosomal aberrations in an S-phase independent manner. The frequencies of polycentric chromosomes were correlated with the dose of the enzyme and the intercellular distribution of aberrations was overdispersed.