Treatment of V79 cells with carbaryl increased the frequency of cells with elevated chromosome numbers (greater than 22). This effect of carbaryl was inhibited by simultaneous addition of glutathione or S9. Although selective forces seemed to act against cells with increased chromosome numbers, such cells were still significantly more frequent in treated compared with control cultures 74 h after treatment. In addition, a high adaptive value for these remaining aneuploid/polyploid cells was indicated because they were able to go through 2 successive rounds of replication immediately before fixation to the same extent as cells with chromosome numbers considered normal (21/22) for this cell line. Multiple chromatid exchanges were also observed after carbaryl treatment. The lack of single exchange events indicates that the effect may be systemic. However, additional experiments are required before this hypothesis can be confirmed or discarded. Considering the results obtained and the possible importance of gene dosage for tumor promotion, it is suggested that carbaryl should be tested for tumor promotion in vivo.