The objectives of this project are (1) to study the biological mechanisms involved in the rescue of defective Friend virus from virus- free Friend virus-induced reticulum cell sarcomas containing the Friend virus genome, (2) to determine which leukemia viruses may act as helper viruses, and (3) determine the role played by the helper viruses in determining the host range of the pseudotypes. Our results thus far indicate (1) that in the presence of helper, Friend virus can be rescued in three hours or less from the virus-free tumor, (2) rescue can only occur in the presence of replicating helper viruses, (3) only members of the FMR group of viruses seem able to act as helpers, (4) N- and B-tropisms are helper dependent, and (5) the ability of the helper virus to replicate is not the only requirement for susceptibility to Friend virus infection.