By focusing in sucrose, gradient L-type pyruvate kinase from human liver could be separated into 2 major forms (pI 6.28 +/- 0.03 and 5.85 +/- 0.09) and a minor more acid form (pI = 5). These different forms could also be detected by focusing in acrylamide-ampholine slab gel. The major forms were interconvertible, the equilibrium being shifted toward the acid form by fructose 1,6-diphosphate and SH reagents, and toward the alkaline form by proteinic factors extracted by ammonium sulphate fractionation from liver extracts and from hemolysates. These factors seemed to be responsible for the stabilization of the liver crude extract enzyme in its alkaline conformation. By acrylamide slab gel electrofocusing, erythrocyte pyruvate kinase from whole hemolysates exhibited a complex pattern composed of at least 3 introconvertible forms. The in vitro aging of the red blood cells and the storage of the hemolysates resulted in a progressive disappearance of the acid forms and in a strengthening of the alkaline form. Partially purified erythrocyte enzyme focused in 2 major bands, interconvertible under the influence of the same factors as those described for L-type pyruvate kinase. Although closely related, the focusing patterns of L-type and erythrocyte-type were never exactly identical. Double immunodiffusion against antihuman erythrocyte-and L-type pyruvate kinases. Moreover, antihuman M2-type serum was unable to neutralize erythrocyte pyruvate kinase as well as to change its electrophoretic mobility. Consequently, we conclude that both human erythrocyte- and liver L-type pyruvate kinases existed under several conformers interconvertible under the influence of the same ligands or proteinic factors; erythrocyte-type enzyme seems to include L-type subunit and not M1- or M2-type subunits. The erythrocyte- and L-type enzymes, however, are not identical and the nature of the differences between them is discussed.