Our data, which extended previous findings about peripheral T cell proliferative capacity to CD4+ and CD8+ subpopulations, suggests that activation ability alone, early in infection in young chicks, may not explain differences in susceptibility between these lines, and that either the numbers of lymphocytes present, interactions with non-lymphocyte cell populations, or the TCR repertoire itself are likely to be responsible for differences in response to MDV infection or development of lymphoid tumors. The gene discussed is CD4; the disease is infection.