Homogeneously staining regions (HSR) and double minutes, two novel chromosome abnormalities, are believed to represent sites of gene amplification. The basic organization of chromatin in an HSR in a human neuroblastoma cell line was shown, by electron microscopy, to be similar to that in a normal chromosome. Double minutes from another human neuroblastoma cell line were found, by dry mass determination, to contain an average of 1.41 X 10(-14) g DNA. This quantity corresponds to 6.38 X 10(3) kilobase DNA per double minute.