To investigate the mechanisms of bovine leukemia virus (BLV)-induced leukemogenesis, we have examined the alterations of the p53 tumor-suppressor gene in sheep and in cattle. The sequences of the open reading frames as well as the intron/exon junctions of the ovine and bovine p53 genes were determined. Pathological samples were screened for the presence of p53 mutations using a single-strand conformational polymorphism assay. Five of ten BLV-induced bovine tumors harbored p53 mutations. In contrast, only one of seven samples corresponding to circulating leukocytes from cattle in persistent lymphocytosis showed an alteration of the p53 gene. Surprisingly, no p53 mutation was found among the 10 BLV-induced sheep tumors analyzed. Altogether, these data indicate that p53 mutations are linked to BLV-induced leukemogenesis in cattle at the transition to the lymphomic stage. These results also enlighten different molecular mechanisms involved in sheep and in cattle during BLV-induced pathogenesis.