Summary. Deformability of erythrocytes from four patients with different types of glucosephosphate isomerase (D‐glucose‐6‐phosphate ketoisomerase, GPI) deficiency has been determined by cell filtration. Young as well as whole erythrocyte populations had a markedly increased rigidity and an abnormally strong attachment of haemoglobin to the inner surface of isolated membranes. Acidic environment may enhance membrane rigidity in vitro and also during passage of the erythrocytes through the spleen. The decrease of deformability at a pH of 6.8 was most pronounced in the splenectomized patients, and likewise in erythrocytes from the splenic artery, which were obtained from one patient during splenectomy. It is suggested that the metabolic environment of the spleen, with its low pH, impairs the deformability of GPI‐deficient erythrocytes and predisposes them to splenic sequestration. The clinical improvement of all patients following splenectomy which is accompanied by an increase of the erythrocyte survival time and by unchanged reticulocyte counts, is in accordance with this view.