Two electrophoretic variants of phosphoglucose isomerase ( PGI) were purified from whole body extracts of DBA/2J and C5 7BL/6Jmice by a substrate-affinity elution from an 8-(6-aminohexyl) amino-A TP-Sepharose column followed by preparative isoelectric focusing. Both PGI variants were shown to be dimers of the same molecular weight, sedimentation coefficient, and Km for fructose-6phosphate, The isoelectric points were found to be 8.4 and 8.7for variants from DBA/2J and C57BL/6J mice, respectively. Differential thermal stability was observed for the two variants in 0.1 M tris-HC1 buffer, pH 8.0, at 54 C; the half-lives of the purified PGI from DBA/2J and C57BL/6J mice were shown to be 3.4 and 1.8 min, respectively, under those conditions. Similar differences were observed for the enzyme variants in the crude homogenates. Antisera against PGI from DBA/2J mice were raised in rabbits. The variants from DBA/2J and C57BL/6J mice showed no significant differences in their respective inactivation curves by the antisera. Results of amino acid composition analyses and peptide mappings of the two PGI variants indicate that the genetic variation of this enzyme might result from a single charged amino acid substitution.