Abstract In a study of 83 breast cancer patients, tumour p53 protein expression was studied by inimunohistochemistry using the monoclonal antibody DO-7. Its relationship with several prognostic variables such as: steroid receptors, cerbB-2 oncogene, Cathepsin D (CD), proliferative indices Ki-67 and PCNA, as well as with expression of the basement membrane proteins collagen type IV and laminin was investigated. p53 protein expression was detected in 51.8% of tumours (43 of 83). Expression was independent of c-erbB-2 overexpression, proliferating index as measured by Ki-67, CD expression and steroid receptor status. Also, p53 accumulation was not correlated with any other conventional clinicopathological parameter considered in this study. A significant positive correlation was found between p53 protein expression and collagen type IV expression ( P < 0.05), and PCNA-LI ( P < 0.05). These data suggest that p53 protein could play an important role in the expansion of breast cancer and in proliferative activity, as determined by PCNA-LI. The results do not resolve whether detectable p53 protein expression represents a random product in tumours with high collagen type IV expression or whether it is an important feature of the malignant phenotype in the expansion of breast cancer.