Somatic cell hybrids were generated by fusing human (A549) cells, cloned after infection with the feline xenotropic CCC virus, to mouse (3T3) cells which are non-permissive for this virus. Hybrid clones were found to be capable of expressing infectious virus. CCC virus expression, however, was regulated in the hybrid cells in such a way that 20–200 times less virus was released into the culture fluid than by the human parental line. Thus human permissiveness for this virus is co-expressed with murine restriction. Markers for twenty human chromosomes were assayed in the hybrid clones. No single human chromosome was found to be essential and sufficient for CCC virus production, since none of them was consistently present or lost in virus-positive and virus-negative clones, respectively.