The impaired antibody response of neonatally bursectomized chickens was restored by bone marrow cells from normal—but not from bursectomized—syngeneic donors. Chimaerism of donor cells in the recipients was demonstrated using the sex chromosomal marker. Attempts to substitute autologous bursal cell suspensions for the bursa in situ were unsuccessful. It is concluded that B lymphocytes in chicken bone marrow have matured in the bursa.