We investigated the presence of K-ras mutations in the serum of 40 patients with respectable stages of adeno- and large cell lung carcinomas. Mutations in codon 12 of the K-ras gene were examined by enriched PCR method in DNA extracts from surgical specimens and serum samples. K-ras mutations were detected in 20 (51%) of 39 analyzed tumours, and in 7 (35%) of 20 patients with K-ras gene mutation positive tumours, a mutation was found in the serum DNA. We also found K-ras mutation in two (10.5%) of 19 serum samples obtained from patients whose tumours were not found to harbor mutation and in one serum sample from patient without tumour sample available for investigation. All of the 14 control healthy persons were negative for serum DNA K-ras mutation assay. Although our results are preliminary they show that K-ras mutation may be detected in serum of patients suffering from adeno- and large cell lung carcinomas and confirm the suggestion that at least a part of a free-cell extracellular blood DNA in cancer patients has neoplastic origin and may become a noninvasive target for genetic investigations of lung cancer patients.