Following appendectomy and sublethal (450 rads) whole body X-irradiation, reconstitution with autologous appendix germinal centre cells led to excellent germinal centre formation in the recipient's spleen. Heat killing of the cells to be injected completely abolished this phenomenon. Apparently a suspension of appendix germinal centre cells contains a (sub)populations of cells capable of germinal centre formation (Germinal Centre Precursor Cells). Following whole body X-irradiation while shielding the appendix, germinal centre activity could only be detected after antigenic stimulation in otherwise normally regenerated splenic follicular structures. This suggests that the cell population recently derived from appendix germinal centres is able to function once again as a population of germinal centre precursors.