Studies on PGMj and PGM3 isozymes in leucocytes from 110 matings with 383 children and 138 mother-child combinations support the hypothesis that these characters are determined by codominant alleles at two separate loci. The two characters were found to be fully developed in leucocytes at birth. The frequency in Denmark of the least common allele at the two loci is 0.19 for PGMJ and 0.25 for PGM∣. The two loci PGM]^ and PGM3 showed no indication of linkage or association and may therefore be regarded as independent markers. A pedigree of a family with the rare PGM* allele is presente