Evidences that demonstrate or imply the occurrence of DNA-RNA viral interactions in man and animals are reviewed. The concurrent presence of two dissimilar oncogenic viruses influenced responses both in vivo and in vitro. Cell-mediated immune responses to Marek's disease herpesvirus were altered in specific-pathogen-free chickens inoculated as embryos with an avian leukosis virus. The possible roles of immunological factors in the pathogenesis and control of tumor development associated with DNA-RNA viral interactions remain to be investigated.