In this study, the expression of proliferating cell nuclear antigen (PCNA) and p53 proteins was investigated in canine mammary tumours and in healthy tissues by immunohistochemistry using the PC10 anti-PCNA antibody and the DO7 and PAb240 anti-p53 antibodies. A total of 46 cases (19 benign lesions: hyperplasia, adenomas and benign mixed tumours and 27 malignant lesions: carcinomas and malign mixed tumours) were examined. No PCNA and p53 staining was evidenced in healthy mammary tissues. The frequency of nuclear PCNA positive staining was higher among malignant tumours (88.9%) than in benign lesions (52.6%) in which the staining extent was always low (< 10%). By contrast, the proportions of PCNA positive cells were moderate (> 10%) to high (> 50%) in 75% of the positive malignant tumours, mainly in solid adenocarcinomas and in papillar adenocarcinomas at a lesser extent. Whereas the DO7 antibody failed to detect any p53 accumulation, nuclear and cytoplasm p53 staining was evidenced using the Pab240 antibody in 15.8% and 48.1% of benign and malignant tumours respectively. The p53 staining remained weak (< 10%) in benign lesions and was more intense in malignant tumours mainly in adenocarcinomas (66.7% of positive solid adenocarcinomas exhibited more than 10% of positive cells). Moreover, p53 positive tumours were also PCNA positive except for one case. These results show that PCNA and p53 may be useful markers for evaluating malignancy in canine mammary tumours.