The envelope glycoprotein of subgroup E avian leukosis viruses (gpE) was purified from Rous-associated virus type 0 (RAV-0) and used in double-antibody competition radioimmunoassays (RIA) and radioimmune precipitations (RIP). When embryo extracts from various inbred lines of chickens were tested, the results of assays for chick helper factor (chf) and RAV-0 were in complete phenotypic agreement with those of RIA for gpE. The expression of genes at the gs and gp loci varied among inbred lines. Normal line 15I 5 embryos contained gpE but not group-specific (gs) antigens, whereas extracts from virus-free lines 15 1 and 6 1 contained both classes of antigen. Line 15 B embryo extracts were negative for both gpE and gs antigen. Soluble gpE was found in sera of all chf-positive chickens from RAV-0-free lines. Sera from adult line 15 B birds were categorized as: (1) positive for RAV-0 but negative for antibody; (2) negative for RAV-0 but positive for gpE and antibody to gpE; and (3) negative for RAV-0, antibody to gpE, and gpE. Evidence of antibody production was completely correlated in RIP of iodinated gpE and serum neutralizations of subgroup E virus. Results of RIP and gpE RIA indicated that half of the sera from nonviremic line 15 B chickens contained antibodies to gpE as well as gpE.