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Monoclonal antibodies (WCL6) specific for carp Cyprinus carpio thrombocytes were produced by immunizing mice with membrane lysates of IgM-negative peripheral blood leucocytes (PBL) and selected on the negative reaction with B cells. WCL6 was reactive with a membrane molecule of approximately 90 kDa and to a lesser extent with molecules of approximately 95 and 110 kDa. In general, between 30 and 40% of PBL were WCL6 + and appeared to be round to spindle-shaped cells. Immunohistochemical analysis of cryo-sections showed much higher numbers of WCL6 + cells in the spleen than in the pronephros, intestine and thymus. Flow cytometric analysis of cell suspensions isolated from these organs only revealed WCL6 + cells (4-10%) in the spleen. Electron microscopy of immunogold-stained WCL6 + PBL showed round but also some spindle-shaped cells with canalicular and granular structures, and a more irregular and electron-dense nucleus than found in lymphocytes. WCL6 + cells with electrondense pyknotic nuclei (without a clear nuclear envelope) were found also and their frequency increased with the length of the isolation and staining procedure used. In the spleen, several differentiation steps of WCL6 + cells were found and hence the spleen seems to be the thrombopoietic organ in carp. Thrombocytes from blood could be activated with collagen; the collagen-activated cells showed a higher side (90 ) scatter by flow cytometric analysis and finally considerable cell death. 1996 The Fisheries Society of the British Isles