Using monoclonal antibody PAb 1801, p53 protein was detected in the neoplastic cells of 39 (46.9%) of 83 colorectal carcinomas studied. Patients with p53+ tumors showed a higher incidence of lymph node and liver metastases (p = 0.035); in patients whose tumors were located in the rectosigmoid, p53 expression also correlated with a more advanced stage according to Dukes' classification (p = 0.015) as well as nodal (p = 0.006) and liver (p = 0.019) metastases. Following amplification of exons 5 to 8 of the p53 gene by means of the polymerase chain reaction technique, single-strand conformation polymorphism analysis disclosed an anomalous migration pattern in 23 of the 39 p53+ tumors and in four of the 35 p53- tumors analyzed. Sequence analysis showed G:C-->A:T transitions in 63.6%, G:C-->T:A and G:C-->C:G transversions in 18.2%, deletions and insertions in 13.6%, and A:T-->G:C transitions in 4.6% of the cases. Loss of heterozygosity was studied in the DNA of 79 patients; allelic loss was found in 29 (49.1%) of the 59 informative patients. Loss of heterozygosity was correlated with p53 overexpression (p = 0.0002) as well as with the presence of mutations as detected by single strand conformation polymorphism analysis (p = 0.0024).