Five of six human squamous cell carcinoma (SCC) cell lines characterized as radiation sensitive (SQ-38, SCC-9, SQ-9G) or radiation resistant (SQ-20B, SCC-35, JSQ-3) exhibited alterations of the p53 gene. The point mutations and a deletion were detected by using single-stranded conformational polymorphism analysis and polymerase chain reaction-direct sequencing. Interestingly, three of three radiation-sensitive and two of three radiation-resistant cell lines revealed mutations in the p53 gene. Point mutations were located in exons 4, 6, and 8 (at codons 72 and 298 in JSQ-3; 273 in SCC-35; 196 in SQ-38), and deletions consisted of 32 base pairs between codons 274 and 285 in SCC-9 and 1 base pair at codon 271 in SQ-9G. Three mutations resulted in substitutions for an arginine residue. Immunocytochemical analysis confirmed p53 protein overexpression in SCC-35 cells which contained a missense mutation at codon 273. In contrast to previous studies which linked alterations in ras, myc, and raf expression with radiation resistance, this study indicates that mutations in the tumor suppressor gene, p53, do not directly correlate with such resistance.