Nicotinamide nucleotide transhydrogenase from Pseudomonas aeruginosa was purified to apparent homogeneity with an improved method employing affinity chromatography on N6-(6-aminoyl)-adenosine-2′,5′-bisphosphate-Sepharose 4B. Polyacrylamide gel electrophoresis of the purified transhydrogenase carried out in the presence of sodium dodecyl sulphate, indicated a minimal molecular weight of 55000 ± 2000. The kinetic and regulatory properties of the purified transhydrogenase resembled those of the crude enzyme, i.e., NADPH, adenosine 2′-monophosphate and Ca2+were activators whereas NADP+was inhibitory.  Nicotinamide nucleotide-specific release of binding of the transhydrogenase to N6-(6-aminohexyl)-adenosine-2′,5′-bisphosphate-Sepharose and N6-(6-aminohexyl)-adenosine-5′-monophospliate-Sepharose suggests the presence of at least two separate binding sites for nicotinamide nucleotides, one that is specific for NADP(H) and one that binds both NAD(H) and NADP(H). Binding of transhydrogenase to N6-(6-aminohexyl)-adenosine-2′,5′-bisphosphate-Sepharose and activation of the enzyme by adenosine-2′,5′-bisphosphate showed a marked pH dependence. In contrast, inhibition of the Ca2+-activated enzyme by adenosine 2′,5′-bisphosphate was virtually constant at various pH values. This discrepancy was interpreted to indicate the existence of separate nucleotide-binding effector and active sites.