Djungarian hamster and mouse L-53 cell lines with 150–200-fold increases in resistance to colchicine contained marker chromosomes with a long homogeneously staining region (HSR). In two clones of Djungarian hamster cells, an HSR was located in the long arm of chromosome 4; in another Djungarian hamster cell line and in mouse L-53 cells, an HSR was located in unidentified marker chromosomes. During cultivation in colchicine free medium these cells lost their resistance at the rate of 2–3 x 10–2 cells per population doubling. The loss of resistance was accompanied by the disappearance of the HSR and emergence of possible double minutes (dmins). These observations suggest that colchicine-resistance is the result of gene amplification and support the idea that dmins may be derived from the HSR in these sy