Much information regarding the association of the Epstein-Barr virus (EBV) with infectious mononucleosis, Burkitt's lymphoma and nasopharyngeal carcinoma has been derived from serological test procedures, involving a number of distinct antigen-antibody systems related to EBV. These relationships have been recently reviewed (Henle & Henle, 1972). Tests for neutralizing antibodies have been hampered by the lack of cells fully permissive for EBV. In addition, infectious virus can be obtained at present only from a few carrier lines and it undergoes essentially one abortive cycle of replication in appropriate lymphoblasts with the synthesis of EBV-induced early antigen (EA) and EBV-determined cell membrane antigens (MA), but little if any EB-virus capsid antigens (VCA) (Henle et al. 1970; Gergely, Klein & Ernberg, 1971; Durr et al. 1970). Nevertheless, three procedures for determination of neutralizing antibodies have been described.