Three methods were used in attempts to obtain non-oncogenic avian leukosis virus for possible use as an immunoprophylactic agent for the control of lymphoid leukosis in chickens. These were: 1) isolate a nononcogenic virus from commercial breeder flocks experiencing very little or no lymphoid leukosis; 2) obtain a non-oncogenic recombinant from mixed infection of a strain with low oncogenicity, Rous-associated virus-60 (RAV-60), with RAV-1 or RAV-2 in cell culture; and 3) attempt to attenuate subgroup A avian leukosis virus by serial passage in avian cell culture. Of 43 isolates obtained from field sources, all were pathogenic except one, and its pathogenicity was questionable because of the low amount of virus tested. All 42 clones from mixed infection of highly oncogenic and poorly oncogenic virus and all clones passaged serially in cell culture were oncogenic.