The specificity of cell-mediated immune responses presupposes the existence of an antigen recognition unit associated with those cells responsible for this type of immunity. Evidence is accumulating which indicates the presence of immunoglobulins on thymus-dependent lymphocytes (T cells) 2 and suggests that these may represent the recognition units (1–10). Much of the supporting evidence was obtained from studies in which anti-immunoglobulin reagents inhibited antigen binding by T cells as detected by direct (autoradiography and erythrocyte rosette formation) and indirect (e.g., highly radioactive antigen-induced “suicide”) techniques. Most of these studies were done in mice; in all instances the T cells studied had been associated in vivo with bone marrow dependent (B) cells and their immunoglobulin products. Agammaglobulinemic (Aγ) chickens represent an animal model particularly well suited to studies involving T lymphocytes since they completely lack B cells and circulating immunoglobulins but have normal T cell development and cell mediated immunity.