Lactate dehydrogenase (LDH) was found to lose rapidly its catalytic activity during the process of entrapment in hydrated reversed micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in octane. It was demonstrated that this inactivation was caused by the surfactant which penetrated into the injected enzyme-containing aqueous bulk phase during the short time of mechanical stirring needed to convert the initial biphasic mixture into a monophasic reversed micellar solution. The unfavorable inactivation phenomenon could be efficiently eliminated by the addition of either NADH or pyruvate into the enzyme stock solution prior to its injection into AOT solution in octane. The catalytic activity of substrate-protected LDH in AOT reversed micelles increased with increasing water content of the micellar system, reaching the maximal level above wo = 30 when aqueous inner cores of reversed micelles grew large enough to allow unrestricted accommodation of the enzyme molecule. It is suggested that the employment of substrate-protected enzymes could represent a generally useful approach for producing highly efficient enzyme-containing reversed micellar reaction systems.