We studied 51 paired samples of tissue sections and cytosol extracts from patients with breast cancer. A very high affinity monoclonal antibody to human p53 protein, DO-1, and polyclonal serum CM-1 to p53 protein were used for two site ELISA assays and CM-1 was used for immunohistochemistry to detect p53 protein accumulation in breast cancer samples. Eighteen carcinomas were positive for p53 by tissue staining and ELISA assay. Nineteen tumours were negative by ELISA and immunohistochemistry, and 14 cases with low levels of positive staining by immunohistochemistry were negative by the ELISA assay. A statistically significant correlation has been found between the degree of staining and the amount of p53 protein measured by ELISA (Pearson's correlation coefficient r = 0.59, P < 0.00001). Our ELISA assay offers an alternative approach to evaluating the p53 status of breast biopsy material, using cytosol extracts routinely prepared for steroid hormone receptor assays. This assay should also be of general application to other situations where the level of p53 protein needs to be determined.