c-Ki-ras-2 sequences were visualized in paraffin embedded sections from normal adult human pancreases and 24 carcinomas of pancreas by an in situ hybridization technique. A biotinylated 1 kbp EcoRI fragment of pHiHi3 DNA was used as probe and the oncogene was visualized as one or two large grains of reaction products produced in more than 9% of normal pancreas nuclei by streptavidin-peroxidase complex and diaminobenzidine tetrachloride. Its amplification in pancreatic carcinomas was detected as one or more large grains in 54% of the nuclei. In addition, tumor cells showed small nuclear and cytoplasmic grains scarcely seen in normal pancreatic cells. The differential transcriptional activity of this oncogene in cancer cells and the adjacent normal pancreatic cells on the same section was evident in sections from 5 cases where normal pancreas was present.