Marek's disease virus (MDV) infections normally have an early cytolytic phase in lymphoid organs at 3 to 6 days post-infection followed by a period of latent infection. Little is known about the mechanisms that govern latency with herpesvirus infections, including Marek's disease (MD). To investigate the importance of immunocompetence for either the establishment or the maintenance of latency in MD, immunosuppressive treatments were applied prior to infection with MDV or after latency was established. These included cyclosporin (Cs) or betamethasone (BM) treatments, neonatal thymectomy plus cyclophosphamide treatment (Tx/Cy), and infection at a young age before full competence. The effect of all the treatments was determined by examining tissues and spleen cells for evidence of virus replication before and after cultivation in vitro. Induced (Cs or Tx/Cy treatments) or natural (infection at a young age) incompetence resulted in prolonged and more widespread early cytolytic infection. Immunosuppression by Cs after latency had developed resulted in a reappearance of cytolytic infection in the spleen and it enhanced the cytolytic infection in the thymus and the bursa of Fabricius. After immunosuppression with Cs, cytolytic infection was found mostly in T cells, although many of the virus-positive cells did not have markers for either T cells or B cells. Immunosuppression by BM after latency had developed also resulted in the reappearance of cytolytic infection in the spleen but only at a very low level. These results suggest that immunocompetence is required for the establishment and maintenance of latency.