The human CD20 antigen, a 35kDa cell surface nonglycosylated hydrophobic phoshpoprotein is expressed consistently on almost all human B-cells, and its monoclonal antibody is used for the therapy on human B-cell lymphoma. In the present study, canine CD20 gene was cloned and sequenced, and the expression of CD20 mRNA was investigated in canine peripheral blood mononuclear cells (PBMCs), and lymph nodes from healthy dogs, and canine lymphoma cells. Using canine cDNA as a template, full-length of canine CD20 gene was sequenced by 5'-RACE and 3'-RACE methods. The full-length of the cDNA sequence of canine CD20 was 1239bp encoding 297 amino acids. The amino acid sequences of canine CD20 showed 73 and 68% sequence similarities with those of human and mouse, respectively. Canine CD20 was predicted to contain domains of amino acid sequences consisting of two extracellular domains (EM), four transmembrane domains (TM), and three intracellular domains (IC) as in human CD20. Canine CD20 mRNA was detected in PBMCs and lymph node from healthy dogs, and B-cells of canine lymphoma, but not in T-cell lymphoma cells and non-T and non-B-cell lymphoma cells by RT-PCR analysis. From these results, canine CD20 might be targeted for monoclonal antibody therapy against B-cell lymphoma of dogs.