Selection of mammalian cells in progressively increasing concentrations of methotrexate results in selective amplification of DNA sequences coding for dihydrofolate reductase (tetrahydrofolate dehydrogenase, 5,6,7,8-tetrahydrofolate:NADP+ oxidoreductase, EC 1.5.1.3). In some cell variants the amplified genes are stable with growth in the absence of methotrexate, whereas in other variants the amplified genes are lost from the population. We have previously reported that in a stably amplified variant of Chinese hamster ovary cells, the genes are localized to a single chromosome. Herein we report that in mouse S-180 and L5178Y cell lines unstably amplified dihydrofolate reductase DNA sequences are associated with small, paired chromosomal elements denoted "double minute chromosomes," whereas in stably amplified cells of the same origin, the genes are associated with large chromosomes.