The tropism of naturally occurring murine leukemia virus (MuLV) was investigated in short-term lymphocyte cultures. The tropism of MuLV was readily defined in fibroblast cultures, but not in lymphocyte cultures. Lymphocytes free of infectious MuLV could be infected across the tropism barrier by partially purified MuLv or by in vitro contact with MuLV-producing lymphocytes. Stimulation of lymphocytes was not required for this cross-infection and replication of MuLV. When cross-infected lymphocytes and was specifically associated with lymphocytes were stimulated in vitro by allogeneic cells, they facilitated MuLV infection of ordinarily non-permissive fibroblasts. This phenomenon (transtropism) required antigenically stimulated lymphocytes and was specifically associated with infection of the lymphocyte by MuLV across the tropism barrier. Thus in contrast with the resting lymphocyte, the transformed lymphocyte acquired the ability to disseminate infectious MuLV to nonpermissive cells. These findings suggest a novel relationship between lymphocytes and leukemia viruses. They indicate one mechanism whereby antigenic stimulation may enhance the development of virus-induced lymphoid neoplasms.