C-reactive protein (CRP) expressed on rainbow trout (Oncorhynchus mykiss) lymphocytes was investigated to determine what role CRP might have on lymphocyte function. C-reactive protein was detected on the surface (S-CRP) of 25% of peripheral blood lymphocytes (PBL) and 4% of head kidney lymphocytes (HKL) by flow cytometry of cells labelled with biotinylated rabbit anti-rainbow trout CRP IgG and fluorescent-coupled avidin. Purified CRP, when added to cells in culture, bound to both PBL and HKL. Stimulation of PBL and HKL by concanavalin A, phytohemagglutinin, or lipopolysaccharide increased the percentage of S-CRP-positive cells, which suggests the production of CRP by lymphocytes. Treatment of lymphocytes with anti-rainbow trout CRP IgG and complement decreased the number of S-CRP-positive cells.