Creatine in density-fractionated red cells is not only a useful criterion of cell age but also an excellent indicator of erythropoietic dynamics particularly under the influence of present and past hypoxia. This is documented on patients with cardiac hypoxia. A histogram is proposed which permits the simultaneous evaluation of four parameters: (1) the creatine concentration of each separate fraction, (2) the absolute density of a given cell fraction, (3) the percentage distribution of cells over the whole gradient and within the different fractions and (4) the general position of the red cell population with respect to its median density, which serves to characterize the overall age of a red cell population. Creatine histograms on fractionated red cells are shown to be useful aids for diagnosis and prognosis in hypoxic clinical states. They may also give retrograde information on past stimulation of erythropoiesis.