Maximizing the amount of protein translated per unit mRNA is an important goal in establishing expression systems. The 5' untranslated region (5'-UTR) of mRNA is known to play an important role in determining the rate of translation. The full length 5'-UTR from the Arabidopsis thaliana heat shock protein (HSP) gene,HSPI8.2 gene, was inserted into the cloning site between the cauliflower mosaic virus 35S RNA promoter and beta-glucuronidase (GUS) gene in the pBI221 plasmid. When this construct was transfected into Nicotiana tabacum (tobacco) BY-2 protoplasts, the level of protein was about 10-fold higher than that of unmodified pB1221. The accumulation of each transcript was the same level. We also demonstrated that the 5'-UTR of the HSP18.2 gene enhances the rate of translation in stable transgenic BY-2 clones and Arabidopsis T87 protoplasts. The 5'-UTRs of the other Arabidopsis HSP genes -HSP17.4, HSP81-1,HSP81-2, andHSP81-3 - also conferred efficient translation. These 5'-UTRs ofHSP genes may be of use in increasing the expression of foreign proteins. In combination with a strong promoter, it can be used in the development of efficient protein production systems.