A carp leucocyte cell line (CLC), originating from peripheral blood, was characterised to assess its suitability for studies into carp macrophage functions. The cells reacted with a monoclonal antibody raised against carp head kidney macrophages. Other macrophage characteristics observed were: binding to an antibody against complement component C3, suggesting that the cells expose a complement receptor; the presence of acid-phosphatase positive cytoplasmic granules; the capacity to phagocytose sheep red blood cells and the upregulation of respiratory burst activity by phorbol myristate acetate, by lipopolysaccharide and by co-culture with carp peripheral blood leucocytes. Moreover, CLC cells produced and secreted an interleukin-1-like factor, as culture supernatants stimulated proliferation of carp peripheral blood leucocytes and proliferation of the interleukin-1-dependent mouse D10(N4)M T cell line. It is concluded that the CLC cell line is suitable for studies on macrophage activation, respiratory burst activity and may also be useful as a source of interleukin-1.