Peripheral blood monocytes and B cells were isolated from a normal donor, and a portion of the B cells was transformed by the Epstein-Barr virus (EBV). Human leukocyte locus A (HLA) class-I and class-I1 molecules were immunoprecipitated by specific monoclonal antibodies after cell labeling with [3H]mannose. Glycopeptides of HLA molecules were obtained by pronase digestion and were analysed by lectin-affinity chromatography. Complex structures were hydrazinolysed, and their sialic acid content was analysed by ion-exchange chromatography, whereas the high-mannose structures were separated by HPLC. In normal cells, class-I antigens bear principally fucosylated biantennary structures while HLA-DR class-I1 antigens bear bi-, tri-and tetraantennary structures and high-mannose structures. HLA antigens are more sialylated on normal B cells than on normal monocytes. An EBV cell line had a very different pattern of HLA-antigen glycosylation when compared with the original B cells. In the transformed cells, the fractions containing biantennary structures are largely decreased. In contrast, an increase of the tri-and tetra-antennary structure fractions is noticed, particularly in class-I1 molecules, while both triantennary and high-mannose structures are increased in class-I molecules. Moreover, when compared to normal B cells, the complex structures of class-I antigens in the EBV-transformed B-cell line are undersialylated while they are oversialylated in the case of the class-I1 molecules. The human MHC or HLA system is a set of closely linked loci situated on chromosome 6 which encode two classes of molecules. Class-I antigens or HLA-A, HLA-B and HLA-C molecules are composed of a highly polymorphic 45-kDa heavy chain (a) noncovalently associated through its extracellular part with f12-microglobulin, which is a monomorphic 12-kDa chain and is not MHC encoded [7]. These molecules have one glycosylation site on the a chain at Asn86 and none on the f12-microglobulin chain [8]. Class-I1 antigens or HLA-DR, HLA-DP, HLA-DQ molecules consist of a 34-kDa monomorphic CI chain noncovalently associated with a 28-kDa highly polymorphic p chain [9]. A 31-kDa invariant Ii Abbreviations. ConA, concanavalin A; EBV, Epstein-Barr virus; endo-H, endo-P-N-acetylglucosaminidase H; E-PHA, erythroaggluthating phytohemagglutinin; HLA, human leukocyte locus A; LcA, Lens culinaris agglutinin; L-PHA, leukoagglutinating phytohemagglutinin; mAb, monoclonal antibody; MHC, major histocompatibility complex; WGA, wheat-germ agglutinin.Enzyme. Endo-8-N-acetylglucosaminidase H (EC 3.2.1.96).chain, not encoded by the MHC loci, is found to be specifically associated with the a-fl complex during its intracellular transport but is apparently not or minimally expressed at the cell surface [lo, 111. The a chain of class-I1 antigens is known to have two N-linked glycosylation sites at Am82 and Asn122 [12]. One of these sites is substituted with a complex-type structure and the other with a high-mannose one. The fl chain has only one glycosylation site at Asnl9 w...