Complementary DNA fragments which showed differential expression relative to unstressed controls were identified and isolated from human oral keratinocytes exposed to hyperosmotic stress. The up- or downregulation of the expression of nine of these cDNAs in response to osmotic stress was determined by Northern blotting. Sequence analysis showed that clones K-5 and K-46 contained identical sequences. Homology searches revealed that K-13 and K-33 were fragments of unknown genes. Among the upregulated cDNAs, K-16 and K-32 were 94 and 83% identical to chromosome 16 bacterial artificial chromosome (CIT987K-A-418G10) and a cDNA (ai49b01.sl) clone, respectively. Another clone, K-34, encoded a protein 73% identical to Bax epsilon. Among the downregulated genes, K-5/46 and K-45 were 99% identical to the og24d08.s1 cDNA clone and to mitochondrial genes for tRNAs and 12S and 16S ribosomal RNAs, respectively, while K-50 was 100% identical to KIAA0905 protein. The gene expression induced by osmotic stress occurred in parallel with the induction of apoptosis and a reduction in protein biosynthesis. This observation, together with the characteristics of the some of the differentially expressed genes, suggests that among the major events induced in oral keratinocytes by hyperosmotic stress are the induction of apoptosis and a decrease in protein biosynthesis, brought about by upregulation of pro-apoptotic genes and downregulation of genes involved in protein biosynthesis.