Reticuloendotheliosis virus naturally infects and cause lymphoid leukosis in chickens and turkeys. We investigated whether it can infect/transfect a variety of human cells in vitro. Successful infection and transfection were achieved with limited replication of virus. These findings, together with the knowledge that other oncogenic retroviruses of chickens and cattle, viz. The avian leukosis/sarcoma viruses and bovine leukemia virus show similar infectivity/oncogencity in vitro for humans, and the recent demonstration of antibodies to avian leukosis/sarcoma virus and reticuloendotheliosis virus in human sera, suggest that these viruses may have a role in the etiology of certain cancers in humans. There is now urgent need to investigate this, particularly because recent epidemiological studies consistently demonstrate excess of cancers in groups occupationally exposed to these viruses.