We observed double minute chromosomes (dmin) and a homogeneously staining region (HSR) in the same metaphase cells obtained from a retinoblastoma cell line, Y79. All of the 132 metaphases examined contained an HSR on the short arm of chromosome 1(1pHSR) and five cells (3.8%) had two to four dmin. To determine whether 1pHSR and dmin carried amplified N-myc sequences, we performed an in situ hybridization using an N-myc probe. Silver grains clustered on and along the 1pHSR, but not on the dmin. These findings indicate that the HSR on chromosome 1 is associated with amplification of N-myc in Y79 cells.