Congenital dyserythropoietic anemia type II (CDA II) or HEMPAS is a genetic disease caused by plasma membrane abnormality. The enzymic defect of HEMPAS has been suggested to be the lowered activity of N- acetylglucosaminyltransferase II, resulting in lack of polylactosamine formation on proteins and leading to accumulation of polylactosaminyl lipids. In contrast to typical HEMPAS cases, cell-surface labeling of the erythrocytes of a HEMPAS variant G.K. showed an absence of polylactosamines either on proteins or on lipids. Fast-atom bombardment mass spectrometry analysis of G.K.'s erythrocyte glycopeptides detected a series of high mannose-type oligosaccharides, which were not detected in erythrocyte N-glycans of normal cells or of other HEMPAS cases: The former contains polylactosaminoglycans and the latter contains hybrid- type oligosaccharides. Keratansulfate (sulfated polylactosamines) in this patient's serum was abnormally low. The galactosyltransferase activity in microsomal membranes prepared from G.K.'s mononucleated cells was 24% of the normal level, whereas this enzyme activity in G.K.'s serum was comparatively higher than normal. Western blotting of G.K.'s membranes using antigalactosyltransferase antibodies showed that G.K. has reduced amounts of this enzyme present. The results collectively suggest that variant G.K. is defective in polylactosamine synthesis owing to the decreased quantity of the membrane-bound form of galactosyltransferase.