Twenty-five colorectal tumors (rectum 6, left colon 13, right colon 6) were studied with respect to the overexpression of p53 and the activation by point mutation of the Ki-ras oncogene. Single point mutations on codon 12 and codon 13 were analyzed after PCR amplivication, dotblotting and sequential hybridization with 12 different oligonucleotides. The intranuclear concentration of p53 protein was measured by flow cytometry after immunofluorescence staining with monoclonal antibody Pab 421. Twelve tumors were found to significantly overexpress p53 and 6 of them had an activated Ki-ras (5 on codon 12, 1 on codon 13). Of 13 tumors which failed to demonstrate over expression of p53, 8 had an activated Ki-ras (5 on codon 12, 3 on codon 13). In our series, p53 overexpression and ki-ras activation appeared to be independent.