THE haemolysin response of rats given a massive dose of X-rays to the surgically exteriorized spleen a few minutes after antigen injection has been found to be essentially normal although delayed for about 2 days1. It has also been shown that the antibody response in these conditions can be almost completely prevented either by splenectomy during the latent period or by sub-lethal whole body irradiation within a few hours of the local spleen irradiation1. This indicates (a) splenic origin of haemolysin formation and (b) the role of non-splenic lymphoid sites in restoring the antibody response. Histological studies reveal an early repopulation of the locally irradiated spleen by lymphocyte-like cells, followed by the appearance of large pyroninophilic cells and plasma cells2. In order to discover the origin of lymphoid cells repopulating the locally irradiated spleen a series of experiments has been performed, and one of them is reported here.