Genomic DNAs from 4 canine spontaneous lymphoid neoplasms were examined by Southern blot hybridization using 7 kinds of oncogene probes. Analysis using a human c-yes-1 cDNA probe revealed amplification of the proto-oncogene in a DNA sample from a dog with lymphoid leukemia. Its degree was about 4- to 8-fold in comparison with a control DNA sample from a healthy dog. The genomic abnormality of canine c-yes-1-related oncogene may have a role in the tumorigenesis of this neoplasm, although the significance of the structural change in this oncogene remains unclear.