Quail embryo fibroblasts were infected at low multiplicity with avian sarcoma virus, and transformed cells were selected by their ability to form colonies in agar. Five clones that failed to produce focus-forming virus were examined for (i) intactness of the integrated proviral DNA, (ii) intracellular viral RNA production, (iii) intracellular viral antigen production, (iv) production of virus particles, and (v) rescue of a functional src gene and of parental host range determinants by superinfection with Rous-associated virus-60, an avian leukosis virus of subgroup E. Deletions in the integrated viral DNA were apparent in three of the five nonproducer clones. In one clone producing focus-forming virus, analysis of the integrated viral DNA revealed an insertion in the region of the genome that codes for src.