Glucose phosphate isomerase (GPI) enzyme activity was measured in two strains of mice, A/HeNNia1 (Gpi-1a) and C57BL/6NNia1 (Gpi-1b). Seven solid tissues (muscle, brain, heart, kidney, liver, lung, spleen) and three cell types (peripheral blood lymphocytes, spleen lymphocytes, erythrocytes) were analyzed for GPI activity in mice from 6 to 36 months of age. No major changes in enzyme activity with age were found. The most striking observation was that A/HeNNia1 mice showed a significantly higher GPI activity than C57BL/6NNia1 mice in four of the solid tissues (muscle, heart, kidney, spleen) and two of the cell types (peripheral blood lymphocytes, erythrocytes) analyzed.