Four virus-infected avian cell lines have been established in culture. Two of these lines, infected with BAI strain A virus, liberate only small quantities of virus in the culture fluid. The cells retain the ability to induce myeloblastic leukemia when inoculated i.v. into 1- to 2-day-old chicks, but do so less efficiently than freshly obtained myeloblasts. These cells do not appear to be transplantable, since the disease produced is characterized by the presence of myeloblasts that liberate large quantities of virus. The other two cell lines, infected with the MC29 strain of avian leukosis virus, liberate normal levels of infectious virus in the culture fluid. When these cells are inoculated into the wing web of 1- to 2-day-old chicks, tumors develop at the site of inoculation which are detectable as early as 4 to 7 days after challenge. Chromosome studies demonstrate that the four cell lines have karyotypes typical of Gallus domesticus. The myeloblastic cell lines (D.U. 11157 and D.U. 1765) show a reduction in the number of microchromosomes. These cell lines have been carried in continuous culture for various lengths of time, can be frozen, are easily recovered in viable form, and appear to be capable of indefinite growth.