Recombinants of Rous-associated virus-0 and Br21 have been used to localize 5' viral sequences that affect the osteopetrotic potential of avian leukosis viruses. Rous-associated virus-0 is a benign subgroup E virus of endogenous origin that does not cause osteopetrosis. Br21 is a constructed subgroup E virus with high osteopetrotic potential. 5' sequences that affected osteopetrotic potential resided in an 834-bp region near the 5' LTR. Sequence analysis of this region revealed differences between Br21 and RAV-0 in the mRNA leader and codons for MA.