The distribution of oncornavirus DNA sequences in various tissues of normal chickens and of chickens with leukemia or kidney tumors induced by avian myeloblastosis virus (AMV) was analyzed by DNA-RNA hybridization using 35S AMV RNA as a probe. All the tissues from normal chickens which were tested contained the same average cellular concentration of endogenous oncornavirus DNA. In contrast, different tissues from lekemic chickens and from chickens bearing kidney tumors contained different concentrations of AMV homologous DNA: in some tissues there was no increase whereas other tissues acquired additional AMV-specific DNA sequences. The increase was the greatest in tissues which can become neoplastic after infection, such as myeloblasts, erythrocytes, and kidney cells. It was directly demonstrated that DNA from AMV-induced kidney tumor contains AMV sequences which are absent in DNA from normal cells. A similar finding had been previously obtained with leukemic cells (15). 3H-labeled 35S RNA from purified AMV was exhaustively hybridized with an excess of normal chicken DNA to remove all the viral RNA sequences which are complementary to DNA from uninfected cells. The 3H-labeled RNA which failed to hybridize was isolated by hydroxylapatite column chromatography which separates DNA-RNA hybrids from single-stranded RNA. The residual RNA hybridized to chicken kidney tumor DNA but did not rehybridize with normal chicken DNA.