Resistance to 10 microgram/ml of puromycin has been analyzed in V79 Chinese hamster cells. Clones that were isolated in 10 microgram/ml of puromycin and subsequently cultivated in its absence consistently lost their resistance. One clone was analyzed in detail by recloning in the presence and absence of puromycin, and it was found that non-puromycin cultivated subclones also lost their resistance and regained inhibition profiles similar to the V79 parent. Reconstruction experiments between sensitive and resistant cells demonstrated that the yield of mutants was not affected by metabolic cooperation. The mutation rate was calculated to be 1 x 10(-7) per cell generation, and was the same within the limits of statistical error in a colchicine-produced polyploid derivative of the V79 line. Although a number of resistant clones were found to have polyploid karyotypes, the polyploid V79 lines was not more resistant to puromycin, nor did it possess a higher frequency of puromycin resistant cells. Studies employing radiolabeled puromycin established that resistance was due to a lowered uptake of puromycin and that an inverse relationship existed between resistance level and uptake rate.