We have applied direct dideoxy sequencing of DNA fragments amplified in vitro by the polymerase chain reaction to the detection of mutations in exon 1 of the c-K-ras gene in human pancreatic adenocarcinomas. Four fresh frozen primary tumors, one metastatic tumor, and twelve formalin-fixed paraffin embedded tumors were analysed. Only three cases showed a possible mutation in codon 12 in a small population of cells, in contrast to the high frequency reported for this alteration with the RNAase A protection assay and allele-specific oligoxynucleotide hybridization. No major difference in sensitivity was found between DNA sequence analysis, and the latter method. Our results suggest that if c-K-ras mutations are indeed present in pancreatic adenocarcinomas at the high frequency reported by others, they must be confined to a small fraction of the cell population to escape detection by direct sequencing. Such a phenomenon would have implications for c-K-ras mutations in the pathogenesis of pancreatic adenocarcinomas.