Human erythrocytes contain three major hexokinase isoenzymes eluted by DE-52 chromatography between hexokinase type I and type II. Cross-reactivities of these isoenzymes were studied by means of a monospecific rabbit antibody against purified human placenta hexokinase type I. It was shown that the three hexokinase isoenzymes were immunologically related, supporting the concept of a postsynthetic mechanism(s) as their origin.