Optimum specific growth rate for the production of recombinant bovine somatotropin (bST) was investigated in fed-batch E. coli fermentation with a controlled specific growth rate. Maximum specific bST concentration based on cell mass decreases linearly with the controlled specific growth rate. Productivity of bST is expressed by a quadratic equation as a function of the specific growth rate and has a maximum value at mu = 0.23 hr(-1).