This study demonstrates the immunoglobulin(Ig)-binding capacity of Ig-positive carp macrophages employing immunofluorescence and immunogold methods. These methods allow for the characterisation of the Ig-binding cells. After internalisation of fluorescent- or gold-labelled Ig (30 min at room temperature), most macrophages from the hindgut were able to bind added purified carp Ig, which could be demonstrated clearly with a second fluorescent or gold label. In pronephros, an important haemopoietic organ in fish, a limited number of monocyte-like cells also showed Ig binding. Pronephros macrophages and neutrophilic granulocytes appeared to be Ig-negative. The use of goat anti-mouse Ig gold particles bound by carp anti-goat antibodies revealed that, in addition to hindgut macrophages and pronephric monocyte-like cells, some lymphoid cells in both hindgut and pronephros cell suspensions were also able to bind Ig. The classic erythrocyte-antibody rosette assay resulted in a limited number of small rosettes in cell suspensions from both organs.