Two new deficient glucose-phosphate-isomerase (GPI) variants have been described in patients suffering from severe chronic hemolytic anemias. The patients' parents were consanguineous, such that the patients were true homozygotes for the mutated GPI genes. In both cases the main cause of the defect in enzyme activity was molecular instability of the mutated GPI molecules, their catalytic activity being nearly normal. GPI 'Paris' was characterized by a slow electrophoretic migration and, above all, a drastically altered affinity for the substrates glucose-6-phosphate (decreased) and fructose-6-phosphate (increased). GPI 'Enfants malades' exhibited a slightly reduced electrophoretic mobility, an abnormal curve of the activity in function of pH, and an abnormal ratio of maximal velocity in the backward direction (fructose-6-phosphate leads to glucose-6-phosphate) to that in the forward direction (glucose-6-phosphate leads to fructose-6-phosphate). No clear relation could be proved between the kinetic abnormalities of the mutant GPI variants on the one hand and the metabolic changes of the GPI-deficient red cells and the severity of hemolysis on the other. Finally we emphasized the possible role of the impairment of hexosemonophosphate pathway in the reduction of viability of the GPI-deficient red cells.