Terrific broth, a complex medium containing a high content of yeast extract, was chosen to cultivate recombinant Escherichia coli with the plasmid encoding the fusion protein gene of thioredoxin (Trx) and human parathyroid hormone (hPTH). The volumetric yield of Trx-hPTH fusion protein in the culture with Terrific broth reached about 800 mg L-1 after optimization. It was found that high content of yeast extract in Terrific broth promoted cell growth and Trx-hPTH fusion protein production. However, the more interesting fact was confirmed that high content of yeast extract was also responsible for the high-level expression of Trx-hPTH fusion protein without specific inducer addition. Further studies indicated that the expression levels of Trx-hPTH fusion protein without specific inducer addition varied greatly with the content and the source of yeast extract contained in the media. Considering that some proteins are toxic to the host and their continuous expression may result in decreasing plasmid stability and protein yields, one should be cautious in selecting yeast extract in media for cultivating E. coli with plasmids carrying toxic genes under T7 control. (c) 2006 Society of Chemical Industry