The objective of this study was to detect by immunohistochemical means nuclear accumulations of p53 and p21 proteins in testicular tumours of dogs. Intense p53 protein nuclear labelling was shown by each of seven seminomas and one intermediate collision tumour. Moderate or intense immunoreactivity was shown by three Sertoli cell tumours. All the tumours also showed p21 nuclear reactivity in parallel with the p53 reactivity. In contrast, four Leydig cell tumours showed no detectable immunoreactivity. The results suggested that high levels of p53 accumulation were associated with the expression of wild-type p53, which was able to activate the transcription of the p21 gene. In addition, weak p53- and p21-nuclear reactivities were detectable in primary spermatocytes within normal seminiferous tubules.