Drug sensitivity of a heteroploid tumour cell line and three clones isolated from it was studied in tissue culture. Cells of the parent line P0, and of clone P1 were fibroblast-like and pseudo- and hypodiploid. Cones P3 and P4 consisted mostly of epithelial and round cells, respectively, and were hypotetraploid. In dose-response experiments, P1 and P4 clones were more sensitive to dianhydrodulcitol than tp0 or P3 clones. Adriamycin was more toxic to P3 than to the other three cell lines. P3 and P4 clones were more sensitive to vincristine than P0 or P1 clones. N-formyl-leurosine, a new derivative of the Vinca alkaloids, was least toxic to P4 cells, and the other three cell lines were equally sensitive. It was concluded that relative sensitivity of the clones depended on the particular drug. Presence of clones of lower sensitivity in heteroploid cell populations may have some bearing on regrowth of tumours after chemotherapy.