Changes in the structure of the rat spleen and the distribution of immune proteasomes in it during early postnatal development have been studied using double immunofluorescent staining of tissue sections with antibodies to the LMP7 immune proteasome subunit and to specific markers of T and B lymphocytes. It has been shown that the white pulp on postnatal day 5 is not yet colonized by lymphocytes and contains a smaller amount of immune proteasomes than the red pulp. At this stage, T and B lymphocytes concentrate mainly in the red pulp. On day 8, B lymphocytes occupy the marginal zone, while T lymphocytes aggregate into dense strands close to the white pulp. By day 18, T lymphocytes form periarteriolar sheaths in the white pulp, and the contents of immune proteasomes in the red and white pulp become equally high. An increase in the total content of immune proteasomes in the spleen on the third postnatal week was revealed in our previous study by Western blotting. In addition to T and B lymphocytes, immune proteasomes have also been revealed in other spleen cell types, probably in macrophages and reticular cells of the white pulp. Thus, the postnatal development of the spleen is associated with an increase in the contents of immune proteasomes in it.