In a previous study based on the kinetics of virus replication and tumor formation (Gazit et al., 1982), it was shown that the organotropism of lymphoproliferative disease virus (LPDV) is confined to lymphoid tissues. The present paper demonstrates that this organ specificity is controlled at the level of infection and integration, that is, the lymphoid organs, which are the only organs sustaining virus replication, and also the only organs in whose cells integrated LPDV proviruses are detectable. At the same time, the efficiency of virus replication within the various target organs is regulated both at the level of infection and integration and at the level of viral gene transcription.