The stable continuous overproduction of a plasmidencoded protein, beta-lactamase, for at least 50 days by Escherichia coli K-12, RB791(pKN), with release into the culture medium has been demonstrated in two-stage chemostats. The second-stage culture was continuously induced with 0.1 mM IPTG. Continuous expression of beta-lactamase could not be sustained with this strain in a single-stage chemostat because of cell death and selection for lac(-1) cells. Beta-lactamase production in the second stage was sensitive to the second-stage dilution rate and the distribution of the limiting substrate (i.e., glucose) between the first and second stages. The fraction of viable, excreting cells and the average copy number in the induced culture was measurably higher under those conditions of dilution rate and substrate distribution which yielded high beta-lactamase levels. The best operating conditions found at 20 degrees C were a first-stage dilution rate of 0.12 h(-1), a second-stage dilution rate of 0.03 h(-1), and equal glucose feed supplied to each stage. Enzymatically active beta-lactamase was produced at a level of 25% of total cellular protein with 90% excretion yielding 300 mg beta-lactamase/L that was 50% pure at an OD(600) < 6.