The leukemogenic mink cell focus-forming (MCF) retroviruses such as MCF 247 have biological properties distinct from those of their ecotropic progenitors. Nucleotide sequences encoding portions of gp70, Prp15E, and the long terminal repeat differ between the two types of viruses. To investigate the role of each of these genetic elements in determining the biological properties of MCF viruses, we prepared infectious molecular clones of MCF 247 and generated a set of recombinants between these clones and a molecular clone of Akv, the ecotropic parent of MCF 247. Each molecular clone of MCF 247 was distinct. All the recombinants between Akv and MCF 247 yielded infectious virus upon transfection. Most interestingly, recombinants which contain the long terminal repeat of MCF 247 were found to have an in vitro host range property that has been correlated with high oncogenic activity and thymotropism of certain MCF isolates; namely, they plated with higher efficiency on SC-1 cells than on NFS mouse embryo cells. Nononcogenic MCF isolates showed a slight preference for NFS cells, whereas Akv virus plated with approximately equal efficiency on the two cell types.