Immunohistochemical and immunoblot analyses were performed on 49 cases of surgically resected primary lung carcinoma to examine the expression of ras oncogene product using monoclonal antibodies to ras-p21. Two different monoclonal antibodies, NCC-RAS-001 and RAP-5 were used for immunohistochemical study. Cancer cells of 16 cases (33%) and 15 cases (31%) were strongly positive for NCC-RAS-001 and RAP-5, respectively. The staining pattern of antibodies was heterogenous among cancer cells, even in the same case. Among various histologic type of lung cancers, squamous cell carcinomas and well-differentiated adenocarcinomas had a tendency to react more intensively than other histologic types of carcinoma. Immunoblot analysis using monoclonal antibody NCC-RAS-004 revealed the presence of ras-p21 not only in cancer tissues but also in non-cancerous tissues in all cases analysed. In 13 cases (27%), cancer tissue expressed more than twice as much ras-p21 as non-cancerous tissues. Our study showed that the over-expression of ras-p21 in lung carcinomas compared with non-cancerous tissues was a relatively common phenomenon, especially in squamous cell carcinomas and adenocarcinomas.