The induction of hsp-70 by gamma irradiation was studied in Chinese hamster ovary (CHO) cells. Synthesis of hsp-70 was assessed by SDS-PAGE of radiolabeled proteins while levels of hsp-70 mRNA were estimated by Northern blot analysis of total RNA. SDS-PAGE analysis indicated that the synthesis of hsp-70 was enhanced in cells exposed to 400 or 1000 Gy. Northern blot analysis of RNA from cells exposed to 1000 Gy indicated a rapid increase in mRNA capable of hybridizing to an hsp-70 oligonucleotide probe. These experiments represent the first report demonstrating that ionizing radiation can increase hsp-70 mRNA.