Expression of the p53 oncoprotein was examined in a wide range of primary human testicular germ-cell tumours using a new mouse monoclonal antibody (MAb) BP53-11 raised and characterized in this study, in parallel with a polyclonal rabbit antiserum CM-1. Immunohistochemistry on paraffin sections showed positive nuclear reaction in at least a fraction of malignant cells in 90 (84%) out of 107 cases studied. Aberrant accumulation of the p53 protein was found among testicular tumours of all major histological types, although generally a higher percentage of positive cases and a higher proportion of p53 over-expressing nuclei within individual lesions was observed in embryonal carcinomas when compared with seminomas. The typical heterogeneous staining pattern characteristic of histological specimens was also found in a cultured cell line derived from a human embryonal carcinoma. In contrast to immunohistochemically undetectable levels in normal testes and morphologically normal tissue areas in the tumour-bearing testes, the accumulation of the p53 protein was clearly identified in a high proportion (59% of cases) of the pre-invasive lesions with positive atypical intratubular germ cells often found in the tissue adjacent to invasive tumours. Altered expression of the p53 protein is therefore a unifying feature of the majority of invasive male germ-cell tumours and the change resulting in high levels of p53 appears to be a relatively early step in the human testicular cancer pathogenesis.