Genetic mechanisms of resistance to disease in 1930 pullets and 296 cockerels were investigated by using a Leghorn line (G), a heavy-breed line (H), and their reciprocal crosses G x H and H x G plus two synthetic lines, GnH and HnG which were segregating for line-intact Z chromosomes. 2. The most frequent cause of death was leukosis. 3. Gmale x Hfemale pullets had a significantly higher adult mortality (40-9%) than the reciprocal cross (27-3%). 4. A direct effect of the Z sex chromosome on the incidence of leukosis or on total adult mortality could not be demonstrated. 5. Maternal breed effects and heterosis were relatively more important than Z sex chromosome and autosomal breed effects as causes of both leukosis and total laying house mortality. 6. The heavy-breed line had higher cellular resistance to the leukosis/sarcoma virus subgroups A and B than did the Leghorn line.
