The QIMR‐WIL human leukocyte cell line contains a filtrable factor which transforms foetal human leukocytes and allows establishment of cell lines. The factor resembled members of the Herpesvirus group in its size as determined by filtration and in its sensitivity to high temperature and ether. The transforming activity of the factor was completely neutralized by human sera containing antibody to the herpes‐like virus of the QIMR‐WIL line but not by control human sera without this antibody. It was concluded that the QIMR‐WIL transforming factor was the herpes‐like virus. Similar transformation was effected by filtrates of the Burkitt lymphoma line QIMR‐GOR which also carries herpes‐like virus. The delayed onset of leukocyte proliferation occurring with virus from QIMR‐GOR cells (49 days) compared with virus from QIMR‐WIL cells (23 days) suggests that strain differences exist. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company