In an attempt to qualitatively identify the membrane antigen (MA) complex induced by Epstein-Barr virus (EBV) infection of lymphoblastoid cells, superinfected Raji cells were surface labelled with 125I by the lactoperoxidase method and solubilized with Triton X-100, then the 125I-labelled membrane proteins were precipitated by sera containing high antibody titers to MA. Analysis of these immune precipitates on sodium dodecyl sulfate polyacrylamide gel eletrophoresis identified four major EBV-specific membrane proteins with molecular weights (mol. wt) of 280,000, 250,000, 170,000 and 90,000. Sera from patients with Burkitt's lymphoma (BL), nasopharyngeal carcinoma (NPC) and infectious mononucleosis (IM) and from EBV-infected disease-free individuals showed differential patterns of reactivity to these antigens with some sera only recognizing three or less of the antigens. The results from invesigations with these sera also indicated that these major proteins were not related to EBV-induced viral capsid antigens (VCA) or the virus-associated early antigen (EA) complexes as defined by immunofluorescence. Metabolic labelling of EBV-infected Raji cells with [14C]glucosamine, followed by Triton X-100 solubilization and radioimmune precipitation, identified the 280,000, 250,000 and 90,000 components as glycoproteins. The lactoperoxidase-labelled 170,000 molecular weight component was not significantly glycosylated and, therefore, could not be categorized as a glycoprotein on the basis of this study. In addition, a glycoprotein with a mol. wt of 130,000 was identified by this approach which also appeared to be specified by EBV. The results from these investigations, therefore, indicated that the EBV-induced MA complex was composed of four major glycoproteins and one nonglycosylated high mol. wt protein.