All replicating retroviruses studied to date contain one major structural phosphoprotein which binds preferentially to the homologous viral RNA genome in-vitro. The specificity and evolution and the effects of phosphorylation on this protein-nucleic acid interaction has been studied using a wide variety of related retroviruses. A number of replication-defective sarcoma viruses have been examined for specific phosphoproteins and protein kinase molecules in pseudotype virion particles containing each sarcoma virus. RNA-phosphoprotein binding studies with the Rauscher MuLv p12 subpopulation, separated on the basis of their levels of phosphorylation, suggested that the level of phosphorylation of these molecules regulate the extent and not the specificity of binding. The viral RNA-binding of the phosphoproteins of replicating retroviruses of endogenous mammalian origin represents a specific nucleoprotein interaction which is evolving with the species. This is regulated by phosphorylation and might be involved in the genetic as well as horizontal transmission of these oncogenic viruses.