Expression systems based on bacteriophage lambda vectors provide a useful vehicle to achieve both high segregational stability and productivity in recombinant protein production. In lysogeny which involves integration of the lambda vector into the host chromosome, the stability of the product gene is 100% for at least 75 generations. In the lytic state, the lambda vector multiplies extrachromosomally to yield gene amplification and high product levels. The level of cloned gene product, β-galactosidase, was 15% of total cell protein in the mutant lytic state, a 60-fold amplification over that in the single-copy lysogenic state. By selecting a suitable lambda system, the recombinant product can be either retained in the cell or released into the extracellular medium.