The molecular alterations responsible for the characteristic enzyme abnormalities in pyruvate kinase (PK) deficiency were investigated in two unrelated children homozygous for PK deficiency. Both variant enzymes were characterized according to the recommendations of the International Committee for Standardization in Haematology. Genomic DNA was specifically amplified by the polymerase chain reaction. Normal and mutant alleles of the L-type PK gene were analyzed by nucleotide sequencing. Heterozygosity of the parents was confirmed by allele-specific oligonucleotide hybridization. In PK Linz a C to T base exchange at position 394 of the L-type PK gene was found. As a result, the 132nd amino acid of the mutant enzyme, arginine (CGC), is replaced by cysteine (TGC). The affected amino acid residue is located within the deduced active site of the protein and the enzyme variant shows strongly altered allosteric properties. PK Beirut shows a C for T substitution at position 1058, changing the 353 amino acid from threonine (ACG) to methionine (ATG). In contrast to PK Linz, this amino acid lies outside the deduced substrate binding site and kinetic parameters of PK Beirut are close to normal. Both enzyme variants show a markedly reduced specific activity and thermolability.