The p53 transformation-related gene is located on 17p, a chromosomal segment frequently under-represented in colorectal adenocarcinoma karyotypes. We have developed a flow cytometric method for detection of its gene product on isolated nuclei by indirect immunofluorescence. Criteria for defining the presence of p53 were established, using parameters related to the difference of fluorescence obtained by incubating nuclei with a specific monoclonal antibody (MAB) versus isotypic control. This method allowed simultaneous quantitation of p53 and DNA in tumor nuclei to be made. Twenty-two of the 41 tumors analyzed (54%) were found to overexpress p53 when compared to 13 normal mucosa specimens. Repeated preparations from different fragments of the same tumor gave reproducible results for the 21 cases tested. p53 was detected in a higher proportion of disseminated tumors (64%) compared to localized disease (39%), but the difference did not reach statistical significance (chi 2 = 2.4, p greater than 0.10). Tumors containing aneuploid cell subpopulations were also more frequently positive (65%) than diploid ones (20%), the difference being significant (Fisher's exact test, p less than 0.03). This dual parameter flow cytometric method, evaluating both DNA ploidy and p53 expression, may prove useful in identifying different biological subgroups of colorectal cancer.