The occurrence of the p 53 gene mutation in breast carcinoma tumour cells, leads to the accumulation of mutant p 53 protein types, whose consequence is the loss of the negative regulation normally exercised by the p 53 gene, which is considered to act as a tumour suppressor. It is possible to demonstrate the presence of mutant p 53 protein types in tumour cell nuclei by applying immunohistochemical procedures to paraffin sections (Clon DO 1, Dianova). We tested 482 primary breast carcinomas for the presence of these proteins, and positive immunohistochemical findings for mutant p 53 proteins were recorded in 21.6% of the cases. In another 14.3% of these breast carcinomas, less than 10% of the tumour cells exhibited positive staining. In the other 64.1% of cases, the immunohistochemical findings for p 53 proteins were entirely negative. Independent of the immunohistochemical staining results, we performed a retrospective analysis of the disease course of this group of primary breast carcinomas: it emerged, that p-53-positive breast carcinomas had a significantly less favourable prognosis as compared to primary tumours, which were negative or weakly positive for this protein group. The accumulation of p 53 proteins in tumour cell nuclei is correlated with negative oestrogen- and progesterone-receptor status, as well as with the degree of proliferation exhibited by the breast carcinoma. Such accumulation is, in contrast, unaffected by the tumour stage, its histological grading, menopausal status, and the overexpression of c-erb B2.(ABSTRACT TRUNCATED AT 250 WORDS)