The ability of avian RNA tumor viruses both to infect and transform cells of target and non-target tissue will be examined. In vitro systems for differentiation of hematopoetic cells from primitive yolk sac cells and differentiation of mesenchymal cells from early limb bud tissue will be employed to examine the possible transformation of immature cell types. Particular attention will be focused on the susceptibility of individual cell types both to infection and transformation by particular strains of avian RNA tumor virus. The transformed cells will be analyzed for alterations in the expression of the particular differentiated cell products which characterize their normal counterparts. The mechanism by which these products are affected by cell transformation will be examined.