Replication-defective transforming viruses including one isolate of cat, and two of mouse origin have been shown to encode polyproteins containing components corresponding to the amino terminus of type C viral gag gene as well as nonstructural components. The possibility that these latter components have transforming potential is indicated by studies of transformation-defective mutants. Polyproteins encoded by such studies have been purified to homogeneity and subjected to detailed immunologic and biochemical analysis. In other studies, intracistronic mapping of the gag regions of the genomes of representative type C and type D retroviruses has been accomplished by use of a combination of genetic and immunologic approaches. High frequency genetic recombination within the env genes of diverse retrovirus isolates is shown to involve sequences involved in host cell receptor site recognition and to be a major factor influencing leukemogenicity. In addition, immunoprecipitation assays for structural proteins of Herpes Simplex viruses 1 and 2 have been developed and collaborative efforts to reevaluate the possible role of Herpes Simplex 2 in cervical carcinomas are in progress.