The objectives of the proposed research are 1) To understand as much as we can about the endogenous viruses of BALB/c mice, particularly the N-tropic virus, and 2) To study genetic recombination between the endogenous viruses of BALB/c mice. Over the past year and a half we have been studying the N-tropic virus derived from BALB/c mice. Our studies have revealed several unexpected properties of this virus and, in conjunction with the work of others, suggest that this virus may belong to a new group of murine C-type viruses characterized by being ecotropic, weakly infectious, and XC- (unable to induce formation of XC plaques). This group of viruses may be widespread, yet we know little about their properties or their disease producing potential. More recently we have used another of our N-tropic variants to produce recombinants with a B-tropic virus, also derived from BALB/c mice. We hope that an extension of these studies will further extend our knowledge of the diversity of viruses that can be derived from BALB/c mice, as well as provide a series of recombinant visuses that may prove helpful in mapping proteins and functions, including oncogenesis, of murine leukemia viruses.