Glucose-6-phosphate dehydrogenase (G6PD) A(+) and G6PD A(-) were purified to electrophoretic homogeneity from human male erythrocytes. The steady state kinetics of the binding reaction of NADP+ to the two variants were studied as a function of pH and temperature at a constant ionic strength of 0.01. The interaction coefficient, obtained according to the Hill equation, rises with an increase in pH and temperature. The observed variation of cooperative interaction is interpreted in terms of an increase in the percentage of the dimeric form of the enzyme as pH or temperature increases. The more rapid increase in the interaction coefficient with increase in pH or temperature for G6PD A(-) as compared with G6PD A(+) shows that G6PD A(-) forms dimers at a lower pH or temperature than G6PD A(+). Analysis of the log Vmax versus pH curves and their temperature dependence for the two enzyme variants indicates the participation in the reaction mechanism of sulphydryl groups or imidazolium group of histidine. The two variants show very similar but minor specific and significant differences in kinetic and thermodynamic properties with respect to NADP+ binding. Thus the additional mutation in G6PD A(-) must be responsible for its increased affinity for NADP(+) when compared to G6PD A(+) from which it has been derived. These results are consistent with an earlier report (Babalola O et al (1972) Proc Natl Acad Sci USA 69, 946-950) that enzyme 'deficiency' in vivo is due mainly to a loss of active enzyme molecules, rather than to a decreased activity of each molecule.