Infection of Kirsten sarcoma virus-transformed non-producer BALB-3T3 mouse cells with UV-irradiated mouse cytomegalovirus (MCMV) resulted in activation of a xenotropic type C virus detected by infectious center formation in permissive rat or mink cells. The levels of type C virus activated by MCMV were related to the UV dose applied. Unter optimal conditions the frequencies of activation varied from 3.0 to 8.0 × 10−4. The capacity of MCMV to activate type C virus was abolished by heat inactivation and by neutralization with specific antiserum against MCMV. Virus induction decreased under conditions of cell exposure to hydroxyurea or actinomycin D, inhibitors of DNA and RNA synthesis, respectively, with actinomycin D having a greater inhibitory effect. This suggests that both DNA and RNA synthesis are required for UV-MCMV induction of murine xenotropic virus.