In order to study the precise localization pattern of anti-TNP antibody-forming cells (AFCs) during the early primary immune response against TNP conjugated TD (thymus-dependent) and TI-2 (thymus-independent type-2) antigens, rats received an intravenous injection with either TNP-keyhole limpet haemocyanin (KLH) or with TNP-Ficoll. Anti-TNP AFCs developed in the spleen already at 2 days after injection of the antigens as demonstrated with our immunoenzyme technique for the detection of specific AFCs. In order to obtain information on the relationship between the non-lymphoid cells in the marginal zone (MZ) and the localization of AFCs, simultaneous staining for marginal metallophils and MZ macrophages (MZM) was performed using the monoclonal antibody ED3. AFCs were not found in the marginal zone (MZ), but the bulk of the cells in the white pulp were found in the outer part of the periarteriolar lymphocyte sheaths (PALS) close to the border between PALS and MZ. The precise localization of the anti-TNP AFCs in the outer part of the PALS resembled the localization of marginal metallophils but the latter cells were mainly present in the outer part of the follicles. So, the present results did not indicate a close relationship between marginal zone macrophages or marginal metallophils and anti-TNP AFCs, neither in the immune response to TD antigens nor in that to T1-2 antigens.