In this study we analysed the expression of p53 protein in a total of 143 carcinomas immunohistochemically. These consisted of 34 prostatic adenocarcinomas, 59 lung and 50 breast carcinomas. In 28 cases, an average of 2-3 additional sections from different tumour areas were analysed. Forty-nine of the 143 carcinomas (34%) showed typical nuclear immunoreactivity by immunohistochemical staining with the p53 antibody CM-1. Two of the 34 prostatic carcinomas (6%) were p53 positive while 25 of the 59 lung carcinomas (43%) and 22 of the 50 breast carcinomas (44%) showed positivity for p53. By grade: 49% of grade III tumours, 36% of grade II and 5% of grade I tumours were p53 positive. There were significantly more p53-positive cases in grade II-III tumours than in grade I tumours (P = 0.001) when all tumours were taken into account. Further, there were significantly more p53-positive cases in grade III than in grade I-II tumours (P = 0.001). In lung tumours there were significantly more p53-positive cases in grade II-III tumours than in grade I tumours (P = 0.018). Similarly, there were significantly more p53-positive tumours in grade III breast tumours than in grade I-II tumours (P = 0.003). The low incidence of p53 positivity in prostate carcinomas suggests that mutations of the p53 gene are not as frequent in the neoplastic transformation of these tumours as in lung or breast carcinomas. The association of p53 positivity with tumours of higher grade suggests that p53 mutations lead to tumours of a more aggressive type. The analysis of tumours by multiple sections indicates that p53 positivity is not evenly distributed in tumour tissue. Therefore, analysis of additional tumour areas may reveal positivity some cases, which is not evident if only one section is studied.