Gene amplification is one of the molecular mechanisms possibly involved in the initiation of tumorigenesis. Homogenously staining regions and double minutes have been shown to contain amplified DNA sequences. We report here a case of B-cell non-Hodgkin's lymphoma (NHL) carrying an abnormal chromosome 5 with an hsr-like region at 5q11-->q13. Chromosome "painting" with a chromosome 5-specific library (pBS5) showed that the amplified material was indeed derived from chromosome 5. Further identification of the amplified DNA sequences was performed with a set of YAC probes localized to 5q12-->q14. The amplified region contained genetic loci defined by D5S6, D5S125, D5S112, and D5S39. A three-fold level of DNA amplification was visualized by Southern blotting using a DNA probe from the amplified region. The present results suggest that unknown gene(s) at 5q11-->q13 might be involved in the process of lymphomagenesis when amplification occurs.