This study was aimed at the evaluation of cell proliferation, the p53 tumor-suppressor gene and the apoptosis index by immunohistochemical methods in canine oral papillomatosis. The study material comprised of tumor tissue samples taken from six dogs, which were admitted to the Pathology Department. Choice of immunohistochemical staining was avidin-biotin peroxidase method. Cases of canine oral papillomatosis, determined to have been caused by CPV-1, were found to have a rather high cell proliferation index, Proliferating Cell Nucleus Antigen (PCNA). Furthermore, all cases were immunohistochemically demonstrated to carry a mutant p53 gene. Despite the mutation of the p53 gene, the shift in the Bax/Bcl-2 ratio of the dogs diagnosed with tumor was in favor of the proapoptotic Bax gene. The apoptotic mechanism was determined to occur through both the caspase-dependent and caspase-independent pathways. While the lesions occupied the entire oral cavity in some cases, histopathologically, malignant transformation was not detected in any of the 6 cases.