Superinfection of chicken embryo fibroblasts transformed by the defective Bryan strain of Rous sarcoma virus (BH-RSV) with two different reticuloendotheliosis viruses (REVs), REV strain T (REV-T) or spleen necrosis virus (SNV), resulted in the production of infectious sarcoma virus pseudotypes. These pseudotypes were neutralized by antiserum prepared against SNV and were unable to infect chicken cells preinfected with either REV-T or SNV. These results suggest that defective BH-RSV is able to use the glycoprotein from REV to form infectious pseudotypes. On the other hand, neither REV-T nor SNV was able to supply a functional reverse transcriptase to the polymerase-negative mutant BH-RSValpha, nor was REV-T or SNV able to complement the defect in the internal protein gene of the temperature-sensitive avian sarcoma virus mutant NY45.