The aim of the present study was to evaluate the influence of intestinal material on the maturing of B lymphocytes in the bursa of Fabricius and on the antibody response to SRBC. Experiments were carried out on chicks whose burso-cloacal stalk had been ligated at hatching, in order to avoid any contact between the bursal tissues and intestinal material. Chicks subjected to a sham operation, or not operated at all, were used as controls. Results obtained by immunoperoxidase test, using an anti-chicken Ig antiserum, indicate that burso-cloacal stalk ligature leads to an increase in B cells in the bursa of Fabricius 20 days after the operation and a slight decrease after 60 days. The response to SRBC reveals an increase after 20 days, though no variations are found with respect to controls after 60 days. Thus, B lymphocytes mature independently of intestinal stimuli. Furthermore, observations carried out at the age of 20 days indicate that B lymphocytes disappear almost completely from the area of cloacal lymphoid infiltration in chicks with a ligated bursa. It is hypothesised that together with a systemic peripheralisation of B lymphocytes in secondary lymphoid organs and in blood, there exists an intramural local peripheralisation, which is interrupted by ligature of the burso-cloacal stalk.