The use of the Gram-positive bacterium Lactococcus lactis in recombinant protein production has several advantages, including the organism's long history of safe use in food production and the fact that it does not produce endotoxins. Furthermore the current non-dairy L. lactis production strains contain few proteases and can secrete stable recombinant protein to the growth medium. The P170 expression system used for recombinant protein production in L. lactis utilizes an inducible promoter, P170, which is up-regulated as lactate accumulates in the growth medium. We have optimised the components of the expression system, including improved promoter strength, signal peptides and isolation of production strains with increased productivity. Recombinant proteins are produced in a growth medium with no animal-derived components as a simple batch fermentation requiring minimal process control. The accumulation of lactate in the growth medium does, however, inhibit growth and limits the yield from batch and fed-batch processes. We therefore combined the P170 expression system with the REED™ technology, which allows control of lactate concentration by electro-dialysis during fermentation. Using this combination, production of the Staphylococcus aureus nuclease reached 2.5 g L−1.