The appearance of a heat-labile glucose 6-phosphate dehydrogenase fraction can be induced by incubation of human fibroblasts at 4°C and pH 7.4. Using this system we first studied the effects of pH and NADP+ on the reversibility of the alteration. In young cells, the induced alteration of glucose 6-phosphate dehydrogenase disappeared if the pH of the medium was lowered to 6.5 or if NADP+ was added. In old cells, the disappearance of the natural heat-labile glucose 6-phosphate dehydrogenase was possible only if both pH 6.5 and NADP* were present. Secondly, we studied the effects of the alteration and its reversibility on the the equilibrium between the enzyme subunits; we showed that alteration and inactivation are linked to a dissociation of the enzyme subunits into inactive monomers. We also proposed a model where the heat-labile enzyme is a transient form between the active dimer and inactive monomer. Our results are in perfect agreement with the theories of post-translational modifications now proposed to explain the presence of the altered enzymes in old cells.