In an attempt to find a low-cost promoter system which could be used for producing recombinant proteins, a low oxygen inducible hmp promoter system in Bacillus subtilis (LAB1886) was characterized by studying the effects of cell density (OD600d), glucose concentration, pH, dissolved oxygen (DO) level, nitrate and nitrite concentration on cell growth and beta-galactosidase expression. The optimal cultivation strategy was found to be to grow the cells in the presence of nitrate to OD600 = 0.2 at a high DO level (80%), and then to induce the hmp promoter by lowering the DO level to 1-2%. As a result, the specific beta-galactosidase activity increased continuously to the maximal value of 1750 Miller Units. Nitrite, as the apparent inducer of the hmp promoter, was produced from nitrate reduction, and had profound effects on cell growth and beta-galactosidase expression. (c) 2006 Society of Chemical Industry.