A set of DNA clones comprising 48 independent HindIII fragments (215 kilobases of sequence) was derived from the N-myc amplification unit of the neuroblastoma cell line NGP. These clones were used to investigate N-myc amplification units in NGP cells and 12 primary neuroblastoma tumors. Three parameters were evaluated: (i) the number of rearrangements from germ line configuration that had occurred during the amplification process; (ii) the homogeneity of amplification units within individual tumors; and (iii) the conservation of amplified sequences among different tumors. The results indicated that remarkably few rearrangements had occurred during amplification, that the amplification units within any one tumor were quite homogeneous, and that although each tumor contained a unique pattern of amplified DNA fragments, there was considerable similarity between the amplification units of different tumors. In particular, the amplification units were strikingly similar over a contiguous domain of at least 140 kilobases surrounding the N-myc structural gene.