Denaturing gradient gel electrophoreses of polymerase chain reaction amplified DNA products and subsequent direct sequencing identified a G-to-A transition causing a replacement of Gly 122 with Arg in an electrophoretic mobility variant of human triosephosphate isomerase, TPI-Manchester. This was the only TPI electromorph variant detected in screening of greater than 3,400 humans in an Ann Arbor, Mich. population. This substitution is at the amino terminus or solvent interaction end of the fifth beta sheet of the alpha/beta barrel structure. The TPI-Manchester variant is a thermolabile enzyme, but the stability of the variant enzyme is not sensitive to other denaturants. This amino acid substitution does not involve residues of the active site and does not detectably alter the kinetic properties of the enzyme. The data provide additional insight into the amino acid residues that are important for the maintenance of the structural characteristics of this very evolutionary constrained protein.