Avian lymphoid leukosis can be induced by lymphoid leukosis viruses belonging to Subgroups A, B, C, and D. The endogenous virus of the chicken (Rous-associated virus type 0) belongs to Subgroup E and has little, if any, potential for inducing lymphoid leukosis. Nearly all chicken flocks are infected with Subgroup A lymphoid leukosis virus. This virus can be transmitted from dam to offspring or by contact with infected birds. Early infection, either by congential means, or soon after hatching, leads to the highest incidence of lymphoid leukosis. Maternal antibody or genetic resistance to infection delays or prevents infection, leading to a lower incidence of disease. In flocks segregating for genetic resistance to infection, continued infection is maintained through dynamic interactions between genetic resistance, acquired or maternal antibody ,and virus infection. Expression of endogenous viral information is controlled by dominant genes, but spontaneously produced Rous-associated virus type 0 can spread through a susceptible flock and be transmitted like an exogenous virus.