Rabbits were intravenously primed and boosted with trinitrophenyl-keyhole limpet hemocyanin (TNP-KLH) and human serum albumin (HSA); both antigens were injected simultaneously. The localization of anti-TNP-antibody-forming cells (AFCs) and anti-HSA-AFCs was determined in various lymphoid organs of the rabbit. In all lymphoid organs of primed rabbits anti-TNP-AFCs outnumbered anti-HSA-AFCs, with the exception of the thymus, in which neither of them was encountered. In the spleen the antibody-forming cells were mainly situated in the periphery of the periarteriolar lymphocyte sheaths (PALS) and in the coaxial sheaths of lymphoid tissue surrounding the terminal arterioles. In the lymphoepithelial organs AFCs were almost exclusively situated in the interfollicular areas, and in the lymph nodes largely in the medulla. An intravenous booster injection led to a secondary immune response (i.e., increase of AFCs) in the spleen. No visible change in the number of specific AFCs was observed in the lymphoepithelial organs. However, in the mesenteric and popliteal lymph node the number of anti-TNP-AFCs had increased tremendously.