In a 13-year-old German girl a GPI deficiency was found to be the cause of a chronic nonspherocytic hemolytic anemia with recurrent hemolytic crises. The hemolytic crises usually occurred after a feverish infection. Only once did the patient require blood transfusion during a crisis. Examination of the family indicated that the patient is doubly heterozygous for the deficiency. The investigation of the biochemical properties of the deficient enzyme revealed an altered electrophoretic migration, a pronounced thermolability, an increased affinity for G-6-P and slightly changed pH optima for both substrates. The described properties of the deficient GPI indicate that we are dealing with a new variant designated GPI-Kaiserlautern.