B-like lymphoblastoid cell lines were obtained by long-term culture of human spleen leukocytes in RPMI 1640 medium containing human plasma fraction instead of whole foetal calf serum. These cell lines, which did not form E-rosettes had membrane immunoglobulins, and expressed Epstein-Barr virus antigens. Most synthesized intracytoplasmic immunoglobulins and were shown to be diploid and to remain so after subcultures. All produced interferon upon induction with Sendai virus.