The "priming effect" is studied to ascertain whether a regulatory system of local immune response exists and, if it does, what its function is. Bursectomized, bursa-ligated and silica-treated chicks were used, together with control birds (silica selectively damages the follicle associated epithelial -FAE- cells). Agglutinins and direct plaque forming cells (PFC)/spleen were checked. The results confirm that contact between the antigen and the bursal tissue acts as a primary stimulus for immune response and that extrabursal lymphoid sites are unable to act in the same way as the bursa of Fabricius. It seems quite probable, then, that a regulatory system of local immune response exists.