Preparative SDS-polyacrylamide gel electrophoresis has been used in the purification of the gp340 component of Epstein-Barr (EB) virus-determined membrane antigen (MA), in tractable quantities, from the B95-8 marmoset lymphoblastoid cell line. Successful renaturation of the purified molecule was achieved. This procedure gave a 50-fold increase in the recovery of antigen compared to conventional techniques. The data suggest that the antigenic sites recognized by human sera containing antibodies to MA are largely confined to the protein portion of the molecule. An eightfold improvement in the yield of gp340 was obtained when B95-8 cells were cultured in the presence of 12-O-tetradecanoyl-phorbol-13-acetate. Gel filtration studies indicate that the major polypeptide components of MA are not associated in detergent solution. Immunization of rabbits with purified and renatured gp340 resulted in the generation of high-titre antisera which were specific for gp340, demonstrating that antigen prepared by this procedure is suitable for further evaluation as an experimental vaccine against EB virus infection.