The frequency of oncogene Ki-ras12 (Val), Ki-ras13 (Cys) mutation was studied in paraffin embedded sections obtained from 50 human lung cancer by in vitro amplification of target sequences via polymerase chain reaction (PCR)and selective oligonucleotide hybridization. 13 samples (26%) contained Ki-ras mutation. 3 carcinomas exhibited 2 distint Ki-ras mutation. The Ki-ras mutation was present in adenocarcinoma, squamous cell carcinoma, adenosquamous carcinoma, well differentiated, moderately differentiated, poorly differentiated human lung cancers.