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The ontogeny of carp (Cyprinus carpio L.) immune cells was studied in mucosal organs (intestine, gills and skin) using the monoclonal antibodies WCL38 (intraepithelial lymphocytes), WCL15 (monocytes/macrophages) and WCI12 (B cells). In addition, recombination activating gene 1 expression was examined in the intestine with real time quantitative PCR and in situ hybridization to investigate extrathymic generation of lymphocytes. WCL38 + intraepithelial lymphocytes (putative T cells) appeared in the intestine at 3 days postfertilization (dpf), which is shortly after hatching but before feeding, implying an important function at early age. These lymphoid cells appear in the intestine before the observation of the first thymocytes at 3-4 dpf, and together with the expression of recombination activating gene 1 in the intestine, suggests that similar to mammals at least part of these cells are generated in the intestine. WCL15 + monocytes/macrophages appeared in the lamina propria of the intestine at 7 dpf, but considerably later in the epithelium, while WCI12 + (B) cells appeared in intestine and gills at 6-7 weeks. From these results it can be concluded that putative T cells occur much earlier than B cells, and that B cells appear much later in the mucosae than in other internal lymphoid organs (2 wpf).