Escherichia coli JM109 strain harbouring repression-, protection- and production-plasmids was cultivated in a two stage continuous stirred tank reactor (CSTR) on complex media by continuously supplementing the second stage with a protein containing medium. In the first stage the cells were propagated. In the second stage gene expression was induced by temperature increase and the product (fusion protein EcoRI::SPA) was formed. The most important process variables were monitored on-line and off-line as appropriate. The glucose and acetate concentrations, yield coefficients of growth and acetate formation, vitality of the induced cells, extracellular beta-lactamase activity, volumetric and specific activities of EcoRI and enzyme productivity were determined as a function of the cultivation time at various dilution rates in the second stage. The dependence of these process variables on the dilution rate at short and long cultivation (low and high generation) times were evaluated and compared with the published data.