The host response to murine ecotropic leukemia viruses is mainly controlled by the mouse Fv-1 gene. This locus controls virus replication at an intracellular stage and prevents provirus integration. Biological studies suggest that the Fv-1 effector molecule recognizes at least one virion structural protein. We have produced host range variants of B-tropic Friend murine leukemia virus in order to study the primary structure of potential viral target proteins. Our results show that conversion of B-tropism to NB-tropism is associated with changes in the primary structure of three gag proteins--p15, p12, and p30. These results suggest that host range conversion is due to a recombinational event, presumably between the parental virus and an endogenous murine virus. They also open the possibility that p12 and p30 may be involved in host range restriction.