Treatment of EB3 cells with the platinum compound, cis-dichloro-diammine-platinum (II), at the 1–10 μM concentration, increased cells reactive in the indirect immunofluorescence test with human sera possessing Epstein-Barr virus (EBV) antibody. This increase was usually twofold to threefold and was observed after 3–7 days. Addition of cytosine arabinoside, at the concentration of 10 μg/ml, or 5-bromo-2′-deoxyuridine, at the concentration of 50 μg/ml, prevented the enhancement; this indicated that DNA synthesis was necessary for the effect. Approximately the same rise in positive cells was observed in 3 EB3 sublines differing in the content of EBV-positive cells. The enhancement was not observed in P3J cells. Immunofluorescence tests with NC37, either treated or untreated, invariably gave negative results.