The expression of the two Epstein-Barr virus (EBV) major membrane proteins gp250/350 (MA-BLLF1) on the surface of recombinant CHO clones cannot be amplified by methotrexate (MTX) selection, perhaps due to toxic effects of these membrane proteins. After removal of sequences encoding the part of the glycoproteins responsible for membrane anchorage, the gp250/350 is secreted into the medium. Following selection with MTX, this construct allows the amplification of the expression products. Besides the possible use of these proteins in protection experiments, they can also be used as antigens for diagnosis, which opens an efficient approach for control of EBV-related neoplasias by early therapy.