Nine new lymphoblastoid cell lines have been established from peripheral blood leukocytes of patients with infectious mononucleosis, subacute myelo-optico-neuropathy (SMON), acute myelogenous leukemia and chronic lymphatic leukemia. All the cell lines grow in suspension and mostly consist of lymphoblastoid cells regardless of the cell source and clinical status of the donors. A few percentage of Epstein-Barr virus (EBV) antigen-positive cells were detected by the immunofluorescence method in all the cell lines. These cell lines have maintained karyotypes of predominantly normal diploid modes. It is suggested that the origin of these cell lines may be certain lymphocytes in peripheral leukocytes. From the present study, a following hypothesis may be offered on the establishment of the lymphoblastoid cell lines. It is conceivable that EBV which is harbored in vivo in lymphocytes might be activated and replicate in the early stage of leukocyte culture. The virus would, thereafter, exert an antigenic stimulation on some of the lymphocytes and consequently they would undergo blastoid transformation, yielding self-sustaining cell lines.