Permanent in vitro growing leukemic cell lines have been established from all types of immunologically classified childhood leukemias. Essential characteristics of primary blasts and cultured cells are identical. In contrast to lymphoblastoid, non-leukemic cell lines, the Epstein-Barr-virus specific nuclear angiten (EBNA) is not detected. Up to now 8 Non-B-non-T cell lines (6 of them were derived from children with acute lymphoblastic leukemia, 2 from patients with chronic myeloid leukemia), 8 T-lines and one B-line have been established. Three Non-B-non-T lines from children with acute lymphoblastic leukemia (KM-3, RU-3, MH-3) and one T-cell line (JM) were cultivated by ourselves. Cultured blasts represent a pure tumor material which can be propagated in large quantities. Leukemic cell lines reveal a new approach for the search after leukemia-associated proteins and represent another possibility for the experimental investigation of the etiology of leukemia.