Individual nutrient salts were experimentally varied to determine the minimum requirements for efficient L (+)-lactate production by recombinant strains of Escherichia coli B. Based on these results, AM1 medium was formulated with low levels of alkali metals (4.5 mM and total salts (4.2 g l(-1)). This medium was equally effective for ethanol production from xylose and lactate production from glucose with average productivities of 18-19 mmol l(-1) h(-1) for both (initial 48 h of fermentation).