Technical Abstract: We have recently described the isolation and molecular characteristics of two recombinant avian leukosis subgroup J viruses (ALV-J) with an ALV-A envelope (r5701A and r6803A). In the present study, we examined the role of the subgroup A envelope in the pathogenesis of these recombinant viruses. Chickens of line 15I5X71 were inoculated at one day of age with r5701A, r6803A, Rous associated virus-1 (RAV-1), or strain ADOL-Hc1 of ALV-J. At 2, 4, 10, 18 and 32 weeks post inoculation (PI), chickens were tested for ALV-induced viremia, shedding and neutralizing antibodies. All except one chicken inoculated with the recombinant viruses (98%) developed neutralizing antibodies by 10 weeks PI compared to only 16 and 46% of the ADOL-Hc1 and RAV-1 inoculated birds, respectively. ALV-induced tumor and mortality in the two groups inoculated with recombinant viruses were different. The incidence of tumors in groups inoculated with r5701A or RAV-1 was 100%, compared with only 9% in the groups inoculated with r6803A or ADOL-Hc1. The data suggest that differences in pathogenicity between the two recombinant viruses might be due to differences in the sequence of the 3' untranslated region (presence or absence of the E element), and therefore, not only the envelope but also other elements of the viral genome play an important role in the pathogenesis of ALV.