Previous findings of low levels of reverse transcriptase (RT) activity in chick cell-derived measles and mumps vaccines showed this activity to be associated with virus particles containing RNA of both subgroup E endogenous avian leukosis viruses (ALV-E) and endogenous avian viruses (EAV). These particles originate from chicken embryonic fibroblast (CEF) substrates used for propagating vaccine strains. To better characterize vaccine-associated ALV-E, we examined the endogenous ALV proviruses (ev loci) present in a White Leghorn CEF substrate pool by restriction fragment length polymorphism. Five ev loci were detected, ev-1, ev-3, ev-6, ev-18, andev-19. Both ev-18 and ev-19 can express infectious ALV-E, while ev-1, ev-3, and ev-6 are defective. We analyzed the full-length sequence of ev-1 and identified an adenosine insertion within the pol RT- region at position 5026, which results in a truncated RT- and integrase. We defined the 1,692-bp deletion in the gag-pol region of ev-3, and we found that in ev-6, sequences from the 5' long terminal repeat to the 5' pol region were absent. Based on the sequences of the ev loci, RT-PCR assays were developed to examine expression of ALV-E particles (EV) in CEF supernatants. Both ev-1- and ev-3-like RNA sequences were identified, as well as two other RNA sequences with intact pol regions, presumably of ev-18 and ev-19 origin. Inoculation of susceptible quail fibroblasts with CEF culture supernatants from both 5-azacytidine-induced and noninduced CEF led to ALV infection, confirming the presence of infectious ALV-E. Our data demonstrate that both defective and nondefective ev loci can be present in CEF vaccine substrates and suggest that both ev classes may contribute to the ALV present in vaccines.