The functional relationship between fish and mammalian thrombocytes is relatively unknown. In this study, a panel of monoclonal antibodies (mAbs) was used to investigate the functional properties of rainbow trout thrombocytes. The mAbs recognize cell-surface molecules on thrombocytes with molecular weights ranging from 17 to 160 kDa. Flow cytometric and immuno-electron microscopic analyses demonstrate that these molecules are expressed at different levels and that surface expression increased upon activation with bovine collagen. Two of these cell-surface molecules (17 and 21 kDa) were directly involved in collagen-induced aggregation of thrombocytes since aggregation was blocked upon pre-treatment with mAbs that recognize the two surface markers. Interestingly, the percentage of thrombocytes in blood increased after stimulation using different antigens. The transcriptional profile of trout thrombocytes was then examined after immuno-magnetic enrichment using the described mAbs to assess potential roles of trout thrombocytes in immune functions. Trout thrombocytes express components of the MHC class Ia pathway, IL1β, TNFα, TGFβ, the interleukin receptor common γ chain as well as CXC and CC chemokines. MHC class IIB and TNFα were expressed at low levels in resting thrombocytes. No evidence was found for the expression of TCRαβ, Ig heavy chain, CD8α or CK1 mRNA. Taken together, these results suggest that rainbow trout thrombocytes express molecules involved in activation, aggregation and genes encoding proteins, that are involved in antigen presentation and immune regulation. © 2004 Elsevier Ltd. All rights reserved.