3H-Labeled complementary DNA specific for the envelope glycoprotein (env) gene of avian leukosis-sarcoma viruses was isolated by selective nucleic acid hybridization techniques, and used to analyze the expression of the endogenous provirus. The endogenous provirus in certain cell types termed chicken helper factor positive (chf+) can synthesize the envelope glycoprotein. Env DNA sequences were present in both chf+ and chf- cells, but env RNA was detectable only in positive cell types. When these cells were infected with the Bryan strain of Rous sarcoma virus (BH-RSV), a defective virus which is deleted in the env gene, the levels of endogenous env RNA remained unchanged, although exogenous BH-RSV specific RNA was synthesized in very high amounts. Thus, the infecting virus did not appear to influence the expression of the endogenous virus. Likewise, the endogenous virus did not influence the exogenous virus expression, since similar amounts of BH-RSV specific RNA were present in all infected cell types, regardless of the level of endogenous virus expression.