Amplification of genes can sometimes be detected by molecular hybridization but not by cytogenetic methods, suggesting that in some cases the units of amplification may be too small to be detected by light microscopy. The experiments reported here investigate whether submicroscopic amplification units are present in early passages of the human tumor cell lines HL-60 and COLO 320. The results show that such cells do contain submicroscopic, extrachromosomal, supercoiled circular molecules harboring MYC genes. The molecules in HL-60 are approximately 250 kilobase pairs (kbp), while those in COLO 320 are 120-160 kbp. The extrachromosomal molecules in HL-60 are shown to replicate semiconservatively and approximately once in one cell cycle. We propose that these submicroscopic elements are precursors of double-minute chromosomes, the usual extrachromosomal manifestation of gene amplification, since both are structurally similar and replicate autonomously.