The reversible reaction catalyzed by ATP phosphoribosyltransferase favors the pyrophosphorolysis of phosphoribosyl-ATP (PR-ATP). The enzyme is inhibited by PR-ATP. To avoid this problem and measure with confidence initial rates of the transferase, we have purified more than one hundred fold the enzyme PR-ATP pyrophosphohydrolase, which irreversibly converts PR-ATP to PR-AMP. Using this coupled assay, we report on substrate kinetics and histidine inhibition studies of ATP phosphoribosyltransferase of Escherichia coli. 1. In the absence of histidine the variation of initial velocity as a function of ATP or phosphoribosyl pyrophosphate (PRPP) concentration, follows Michaelis-Menten kinetics, with ATP inhibiting at high concentrations. In the presence of histidine a change from hyperbolic to sigmoidal kinetics is observed. 2. Apparently AMP acts as a competitive inhibitor of ATP. 3. The bisubstrate kinetics gives a pattern of parallel lines, suggesting a double displacement mechanism. 4. The inhibition by histidine appears not to be cooperative or perhaps slightly negatively cooperative.