The presence of immunoregulatory cells in chicken thymus was studied by using several different systems. Chickens injected with large numbers of syngeneic thymocytes were tested for their ability to produce antibody to heterologous red cells. Similar chickens were studied for their ability to reject allogeneic skin grafts. In separate studies, mixtures of thymocytes with spleen cells or with peripheral blood leukocytes were assayed for their ability to respond to PHA or to produce a graft-vs-host reaction in embryonic chicks. These studies indicated that immunoregulatory cells exist in chicken thymus, which displays both helper and suppressor activity. The suppressor cells were more prevalent or more easily detectable in young birds and in chickens with intact bursas. The helper function of thymocytes was seen to better advantage with cells derived from older animals and from bursectomized donors.