Amplification of SV40 genes in SV40-transformed Chinese hamster embryo cells (CO631) by chemical carcinogens as well as by herpes simplex virus infection can be inhibited by infection with the defective parvovirus, AAV-5. This is shown by in situ hybridization with SV40 DNA of AAV-5-infected CO631 cells after treatment with herpes simplex virus type 1 or with chemical carcinogens: the initiator-induced selective amplification of SV40 sequences is prevented in the presence of the parvovirus. During HSV-infection and also in the presence of carcinogens, in CO631 cells parvovirus DNA is synthesized and AAV-5-specific antigens are expressed as revealed by hybridization with cloned AAV-5 DNA or by immunofluorescence with monoclonal antibodies, respectively. The inhibition of initiator-induced gene amplification could point to the mechanism of parvovirus-mediated inhibition of tumor development and may indicate an important role of selective gene amplification in oncogenesis.