Six monoclonal antibodies and a polyclonal antibody (CM1) were used to investigate the overexpression of p53 protein by immunohistochemistry (IHC) in six sarcomas and 21 mammary carcinomas from 27 dogs. IHC was compared with p53 gene mutation analysis performed on the same samples. Only the monoclonal PAb240, PAb421 and the CM1 antibodies were able to detect expression of canine p53 protein. CM1 was found to give the highest concordance (8/11) between positive expression of the p53 protein by IHC and the presence of a gene mutation. In the samples that were negative for p53 expression by IHC, but contained a p53 gene mutation according to DNA analysis, the mutation often affected the epitopes that could have been recognized by these antibodies. Only one out of 16 tumours without a p53 gene mutation had a weakly positive IHC result. These findings indicate that in these two types of canine tumours, IHC - particularly with CM1 - can detect many alterations in p53 expression owing to a gene mutation. False-positive results were very infrequent.