A monoclonal antibody against carp macrophages (WCL15) has been utilised in flow cytometry, immuno-histochemistry and immuno-electron microscopy to assess the distribution of monocytes/macrophages in developing carp lymphoid tissues. In suspensions of living cells WCL15 reacted strongly with cytoplasm and plasmic membrane of macrophages. It also cross-reacted with a subpopulation of thrombocytes, but this reaction could be neglected by double immunostaining in combination with a thrombocyte-specific marker. In Bouin-fixed tissues the antibody distinctly recognised macrophages. Macrophages were found from day 2 post-fertilisation in head kidney and in the dorsal portion of the yolk sac epithelium. From 1 week onwards macrophages were found scattered in thymus and gut and during the second week in spleen. Macrophages increased in number in all lymphoid tissues until the 6-8th week post-fertilisation, but they decreased except in thymus, where they became localised mainly in the cortical-medullary boundary, and in white pulp areas of head kidney. The role of macrophages in allowing an early non-specific defence in young fish and in co-operating during the differentiation processes of T-cells and B-cells is discussed.