Mutations in the p53 nuclear oncogene occur frequently in a wide spectrum of human malignancies and the mutant protein may prove to be a useful diagnostic or prognostic marker. It can be detected in fixed tissues by immunohistochemistry, but the type of fixative and conditions of fixation used can introduce variability. For routine clinical use, a method of analysis which is more easily standardized would, therefore, be of benefit. A two-site enzyme-linked immunosorbent assay (ELISA) was used to measure the level of p53 protein in soluble extracts from 20 gastrointestinal cancers (11 colonic, nine gastric). Immunohistochemistry was also performed on the paraffin-embedded sections of these tumours and the results of the two assays were compared. ELISA detected p53 at various levels in 10 cases, all of which were also positive by immunohistochemistry. Of the other 10, eight were immunohistochemically negative but two were positive. When the immunohistochemically positive specimens were ranked by scoring the degree of staining, there was a highly significant correlation with the quantitative ELISA results. Our study shows that the ELISA is sensitive and highly specific. It offers an alternative and simple method of assessing the p53 status in human tissues.