The molecularly cloned, long terminal repeat (LTR) of the Moloney sarcoma virus (M-MSV) provirus has been covalently linked to c-mos, the cellular homolog of the M-MSV-specific sequence, v-mos. These newly constructed clones lack any M-MSV-derived sequences other than the LTR, but in DNA transfection assays they transform cells as efficiently as cloned subgenomic M-MSV fragments containing both v-mos and LTR. Cells transformed by LTR:c-mos hybrid molecules contain additional copies of mos DNA, and several size classes of polyadenylated RNA's with sequence homology to mos. The activation of the transforming potential of c-mos by the proviral LTR suggests a model whereby LTR-like elements could activate other normal cell sequences with oncogenic potential.