A new variant of the PGMa1 erythrocyte enzyme system not identical with the known variants of the system has been detected in the hemolyzed red blood cells of a healthy blood donor by isoelectric focusing. Using this technique the variant is represented by two bands, a strong and slow one more cathodically located than the a3 band and a weak one in the position of the a2 band. Using agarose thin layer or acetate foil electrophoresis the variant is represented only by a minimal cathodic broadening of the PGM1 1 band and therefore it is easily overlooked. Investigation of the propositus' family shows that the variant occurs combined with the common alleles PGMa1(1), PGMa1(2), and PGMa1(3) and that it has an autosomal dominant inheritance. Obviously the variant has a very low frequency.