The methodological aspects of (Na+, K+)-ATPase-dependent uptake of 86Rb, a potassium analog, were examined on human lymphocytes isolated from peripheral blood. The study of the time-course, the kinetic parameters, i.e., maximum velocity (Vmax) and Michaelis constant (Km) and the ouabain inhibition curve of 86Rb+ uptake confirm that circulating lymphocytes represent a suitable model for the study of (Na+,K+)-ATPase in human diseases. An application to human obesity is reported: the results indicate that 86Rb+ uptake on circulating lymphocytes is similar in obese and non-obese subjects. Therefore, (Na+,K+)-ATPase does not seem to be involved in the pathogenesis of human obesity.