Chinese hamster ovary cells were treated with combinations of the restriction endonucleases AluI, EcoRI or PstI and bleomycin, DNase I or X-rays. With few exceptions the frequencies of polycentric chromosomes calculated as dicentric chromosomes (DIC) were additive or less than additive after combinational treatments when compared with the effects of treatments with the single compounds. Less than additivity was found when the frequencies of DIC were higher than 150 DIC per 100 cells indicating a saturation effect. The data suggest that DNA double-strand breaks (DSB) with different end-structures produce polycentric chromosomes to the same extent as DSB with the same end-structures.