The Epstein Barr virus membrane antigen (MA) complex is composed of three major high m.w. glycoproteins designated gp 300/350, gp 200/250, and gp 85/90. In the experiments reported in this paper, the gp 300/350 glycoprotein was purified from MA-positive extracts prepared from the B-95-8 cell line. This was accomplished by tandem combination of ion-exchange and lectin chromatography. It was routinely possible to purify this glycoprotein 400- 600-fold by this procedure. Sera from animals immunized with gp 300/350 purified by this approach neutralized EBV infectivity and mediated antibody-dependent cellular cytotoxicity (ADCC) demonstrating that both of these important antigenic determinants were expressed on this glycoprotein. The presence of an ADCC determinant on this molecule was further substantiated by the finding that monoclonal antibody to gp 300/350 blocked ADCC mediated by a human ADCC antibody-positive reference serum.