The molecular abnormality of a phosphoglycerate kinase variant associated with severe red cell enzyme deficiency ( about 4% of normal) and episodes of hemolysis with jaundice was examined. The Michaelis constants for the substrates and co-enzymes (1.3-diphosphoglycerate, 3-phosphoglycerate, ATP and ADP) were not grossly different from that of normal. However that variant enzyme was very labile in vitro. Nucleotide sequence analysis of the variant cDNA revealed a deletion of codon AAG in exon 7. The codon deletion should result in the election of one of the tandem lysine residues existing at amino acid 190-191 of the enzyme protein. Based on the three dimensional structure of the protein, molecular instability could could be induced by the deletion of a lysine residue.