A simple method for simultaneous comparative observations of immunofluorescence, cellular morphology and autoradiography has been developed, and this method was applied to investigate the early transformation events of human cord blood lymphocytes by Epstein-Barr virus (EBV). The results indicated that transformation occurred sequentially from EBV-determined nuclear antigen (EBNA) synthesis, to blastogenesis, to DNA synthesis, and consistently resulted in unlimited growth of the antigen-positive lymphoblasts. EBNA synthesis was not hampered in the presence of Ara C, but was significantly inhibited by a short-term exposure to cycloheximide immediately after EBV infection. The response of leukemic cells to EBV was further studied. Only leukemic lymphocytes with B-cell markers showed susceptibility to EBV, in close connection with the presence of complement receptor. In contrast to EBV infection of normal B-lymphocytes, EBNA synthesis in three chronic lymphocytic leukemia (CLL) preparations was not followed by either blastogenesis or DNA synthesis and the infected cells shortly resulted in cell degeneration. In case of acute lymphocytic leukemia (ALL), EBNA-positive cells were highly evident and grew rapidly for up to 9 days. The cell proliferation, however, was temporary and death resulted in about 2 weeks. No significant synthesis of EBV-related early antigens and viral capsid antigen was noted in any of the infected culture throughout the incubation period.