Carbohydrate-deficient glycoprotein syndromes (CDGS) are a family of multisystemic congenital diseases resulting in underglycosylated glycoproteins, suggesting defective N-glycan assembly. Fibroblast extracts from two patients with a recently described variant of this disease (CDGS type II) have previously been shown to have over 98% reduced activity of UDP-GlcNAc:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II [GlcNAc-TII; Jaeken, J., Schachter, H., Carchon, H., De Cock, P., Coddeville, B. & Spik, G. (1994) Arch. Dis. Childhood 71, 123-127]. We show in this paper that mononuclear cell extracts from one of these CDGS type-II patients have no detectable GlcNAc-TII activity and that similar extracts from 12 blood relatives of the patient, including his father, mother and brother, have GlcNAc-TII levels 32-67% that of normal levels (average 50.1% +/- 10.7% SD), consistent with an autosomal recessive disease. The poly(N-acetyllactosamine) content of erythrocyte membrane glycoproteins bands 3 and 4.5 of this CDGS patient were estimated, by tomato lectin blotting, to be reduced by 50% relative to samples obtained from blood relatives and normal controls. Similar to patients with hereditary erythroblastic multinuclearity with a positive acidified-serum lysis test (HEMPAS), erythrocyte membrane glycoproteins in the CDGS patient have increased reactivities with concanavalin A, demonstrating the presence of hybrid or oligomannose carbohydrate structures. However, bands 3 and 4.5 in HEMPAS erythrocytes have almost complete lack of poly(N-acetyllactosamine). Furthermore, CDGS type-II patients have a totally different clinical presentation and their erythrocytes do not show the serology typical of HEMPAS, suggesting that the genetic lesions responsible for these two diseases are possibly different.