Abstract SE 21Qlb is a recombinant avian oncovirus which is continuously produced by a line of transformed quail cells. These cells produce no detectable infectious viral progeny. Virions of SE 21Q1b contain 1–2% of the normal amounts of RSV-specific RNA found in PR-RSV-C virions and substantial quantities of heterogeneously sedimenting nonviral RNA ( M. Linial, E. Medeiros, and W. S. Hayward, 1978b, Cell, 15:1371–1381). RNA extracted from purified virions of SE 21Q1b is as efficient a template for amino acid incorporation in a mRNA-dependent reticulocyte lysate as oligo(dT)-cellulose purified cellular mRNA. Virion RNA from SE 21Q1b serves as a template in vitro for a small amount of the precursor to the internal structural proteins ( gag proteins). But unlike RNA from other avian oncoviruses, SE 21Q1b RNA makes, in addition, a number of proteins whose sizes on sodium dodecyl sulfate (SDS)-polyacrylamide gels resemble those of a number of uninfected quail fibroblast proteins. The proteins range in size from 15,000–220,000 daltons. Immunoprecipitation of proteins synthesized in vitro from SE 21Qlb virion RNA with antisera against the gag, pol, and env gene products shows that this RNA serves as a template for only one viral gene product, the recombinant gag gene precursor of 72,000 daltons (ΔPr76) ( R. Shaikh, M. Linial, J. Coffin, and R. Eisenman, 1978, Virology 87, 326–338). Addition of lysed virus to proteins made in vitro from PR-C RNA or RNA from PR-E-95c, a recombinant whose gag gene structure is similar to that of PR-E-21Qlb, results in the cleavage of gag protein precursors ( K. von der Helm, 1977, Proc. Nat. Acad. Sci. USA 74, 911–915) into several of the internal structural proteins. On the other hand, products of translation of PR-E-21Q1b RNA are not detectably cleaved into the internal structural proteins, suggesting that a very low amount of PR-E-21Q1b translation products are gag related. A protein of 37,000 daltons constitutes about 15% of the total protein synthesized from SE 21Qlb RNA. Data from serological and peptide mapping studies indicate that this protein is unrelated to the virion gag, pol, or env proteins. However, the major tryptic peptide of this protein appears to be identical to the major peptide of a prominent quail cell protein having the same apparent molecular weight as the in vitro translation product. Thus, several lines of evidence suggest that SE 21Q1b virions contain substantial amounts of functional cellular messenger RNAs.