We have analyzed the inducible synthesis of recombinant porcine somatotropin (rPST) from the phage T7 gene 10 promotor on the vector pET3a [Rosenberg et al., Gene 56 (1987) 125-135]. Low-level synthesis of phage T7 lysozyme is crucial for high-level synthesis (40%) of rPST, which is greatly reduced if T7 lysozyme synthesis is absent or too high. The synthesis of rPST mRNA is optimized in those constructs coding for low levels of T7 lysozyme, with a reduction in mRNA levels in constructs coding for higher levels of T7 lysozyme or no lysozyme. The rPST can be readily purified following a single chromatographic step and is biologically active as determined by the tibia test following administration to hypophysectomized rats.