The location and temporal appearance of the different classes of lymphocytes were investigated in the developing white pulp of the rat spleen. Additionally, indications were sought for the involvement of non-lymphoid cells in the localization of lymphocytes. The lymphocytes were demonstrated by their surface determinants (W3/13, IgM, IgG, IgA, Ia) in a two-step immunoperoxidase method; the non-lymphoid cells were characterized by immuno-enzyme-histochemical techniques. The results showed that 1) already at birth strongly Ia-positive cells are present in the T-cell area; 2) the marginal zone develops as a distinct compartment, independent of the PALS and follicles; 3) follicles are recognizable at day 14, the capacity to trap immune complexes on follicular dendritic cells occurring one week later. A possible relation between the development of the follicles and the differentiation of the follicular dendritic cell is discussed.