The concept of permanently integrated type C viral genes in somatic cells is well established. Their expression may be under tight genetic control. Relaxation of this control appears related to onset of neoplasia and cell transformation. Thus, it is important to understand the relations among various virogenes and genes which code for viral related functions. The principal method of this investigation is that of interspecies somatic cell hybridization. Hybrid cells are constructed from a variety of primary and established cell lines such that chromosomes of one parent are preferentially or exclusively lost. Isozyme and karyologic analysis allows viral genes and genes for viral related functions, such as receptors to be assigned to specific chromosomes. The results show that it is possible to construct the following classes of hybrid cells suitable for mapping integration sites of infectious and endogenous viruses: (1) rodent X primate (including human); (2) Mus cervicolor and Mus caroli X Mus musculus and Chinese hamster; (3) Mus musculus X Chinese hamster. All of these hybrid cells are of course useful for formal genetic studies. An ecotropic receptor has been assigned to a murine chromosome in M. cervicolor and M. musculus.