Combined quinacrine fluorescent (Q-banding) and centromeric heterochromatin (C-banding) staining of metaphase chromosomes from the T199H heterozygous translocation stock of Mus musculus has shown that the break points in chromosomes 10 and 13 are located closer to each centromere than previously believed. Virtually all of chromosome 13, including the C-banding material, has been translocated to the distal region of chromosome 10. The presence of C-banding material in an interstitial location in the longer of the two translocation chromosomes provides a distinctive mitotic chromosome marker. Both the C-banding region and the euchromatic region of chromosome 13 are longer in the translocation chromosome than in the normal homolog. During meiosis in heterozygous males a chain of four chromosomes is the most common quadrivalent configuration. In the meiotic chromosome the amount of interstitial C-banding is markedly reduced from that seen in mitosis. C-banding is seen at the centromeric region of both the short translocation chromosome and the Y during meiosis but not mitosis.