One of the most common cellular gene which negatively regulates the cell cycle, thus functioning as tumour suppressor gene, is the p-53 gene. The presence of this mutated gene has been correlated with, the aggressiveness of several malignant neoplasmas. Expression of the p-53 gene product protein was screened in 55 untreated human germ cell testicular tumours, furthermore a relationship between p-53 expression and clinical resistance was investigated. Using monoclonal antibody and immunoenzyme staining elevated p-53 level could be demonstrated in nuclei of embryonal carcinoma (84%) and seminoma components (56%). Most of the choriocarcinoma cases showed positive staining. Teratomas expressed this antigen negatively or scarcely. In seminomas the highest level of p-53 was stated in stage I. In contrast the opposite tendency could be demonstrated in embryonal carcinomas where p-53 was ++ positive in stage III. Between the high level of p-53 and clinical resistance a converse correlation could be stated because the resistant tumours expressed no or low, the sensitive tumours high level of p-53 protein (P 0.01). These results suggest that elevated p-53 expression could be a prognostic marker of sensitivity in testis cancer.