The enzymatic activity of G3PD [glyceraldehyde-3-phosphate dehydrogenase] and LDH-B [lactate dehydrogenase] in erythrocytes was assayed in 3 patients carrying a rearrangement of the short arm of chromosome 12. The 1st patient is trisomic for the whole short arm. The 2nd is trisomic only for the distal part and the 3rd is monosomic for the proximal part. G3PD activity increased in patients 1 and 2 and normal in patient 3. LDH-B activity increased in patients 1 and 2 and decreased in patient 3. The erythrocytic G3PD locus is localized on the distal part of 12p, between 12p12.2 and 12pter, and the LDH-B locus is on the middle 3rd between 12p12.1 and 12p12.2.