Phosphoglucose isomerase (PGI) and 16 other biochemical genetic markers were studied in an Israeli-Arab family previously described for hereditary deficiency of adenylate kinase (AK) and glucose 6-phosphate dehydrogenase (G6PD). In this inbred family a rare PGI*3 allele was observed in 11 of 32 members tested, indicating an autosomal codominant inheritance. The electrophoretic mobility of this allele is similar to that of the PGI*3 allele found in Indian populations, but unlike the Indian allele, it has a very low specific activity and heat stability. This PGI*3 allele, designated PGI*3 (Israel), seems to be a different unstable mutation and along with AK and G6PD deficiencies seems to be associated with severe anaemia.