Previously we have reported non-random cytogenetic abnormalities involving the short arm of chromosome 9 (9P) in the majority of primary non-small cell lung cancer (NSCLC) patient samples, which indicated loss of DNA sequences. In another lung tumor, pleural malignant mesothelioma (MM), cytogenetic changes also include apparent deletions of 9p. To define the location and extent of deletions of 9p in NSCLC and MM, Southern blot analyses on six NSCLC and five MM cell lines using molecular probes to 9p loci (IFNA, IFNB1, D9S3, and D9S19) were performed, and DNA dosage was determined by densitometry. Our data demonstrated reduced dosage of 9p sequences in three of six NSCLC and four of five MM lines. A homozygous deletion of D9S3 was found in one NSCLC and one MM cell line. The region of common loss overlapped the D9S3 locus and was flanked by the IFNB1 and D9S19 loci. IFNB as previously been localized to 9p22, and the D9S3 and D9S19 loci have been mapped in this study by in situ hybridization to 9p21 and 9p13, respectively. We hypothesize the existence of one or more tumor suppressor genes on 9p with a role in the development or progression of NSCLC and MM.