NAD kinase was purified 180-fold from Bacillus licheniformis to determine the role it plays in NADP turnover in this organism. The enzyme was found to have a pH optimum of 6.8 and an apparent Km for NAD of 2.7 mM. The ATP saturation curve was not hyperbolic; 5.5 mM ATP was required to reach half maximal activity. Both Mn2+ and Ca2+ could be substituted for Mg2+. Several compounds including nicotinic acid, nicotinamide, nicotinamide mononucleotide, quinolinic acid, NADPH, ADP, AMP and cyclic AMP did not affect NAD kinase activity. In contrast, the enzyme was inhibited by NADP at concentrations typically found in logarithmic cells of B. licheniformis. This inhibition was competitive with NAD and had a Ki of 0.13 mM. It is suggested that in vivo NAD kinase activity is highly dependent on the concentrations of NAD and ATP and the proportion of oxidized and reduced NADP.