Pseudodiploid Chinese hamster cells (line CHO) were treatedin vitrofor 2 or 32 h with hexavalent chromium compounds (K2Cr2O7and K2CrO4), at concentrations of 10−5M and 10−6M, in medium containing BUdR, and sister chromatid exchanges (SCEs) in 2nd division metaphases were scored. The frequency of SCEsdoubled in pseudodiploid cells, whereas it only rose by 50% in the pseudotetraploid fraction. Also in cytochalasin B-generated pseudotetraploid cells, 10−6M K2Cr2O7induced less SCEsper chromosome than it did in pseudodiploid cells. These observations suggest that at the concentration of chromium used, the number of active molecules available for interaction with chromosomal sites may be limiting. The ratio of 2nd generation (asymmetrical) to 3rd generation (symmetrical) SCEs, scored in 3rd metaphases, was lowered by continuous K2Cr2O7treatment. This apparent excess of symmetrical SCEsin 3rd metaphases could be explained with the induction of single-strand exchanges by chromium. This possibility was supported by the occurrence of light regions in 1st metaphase chromosomes of chromium-treated cells, as well as both « isolabelling » and « isounlabelling » in 2nd metaphases.