In a previous study, monoclonal antibodies (mAbs) against different haemolymph molecules of the marine shrimp Penaeus monodon were produced and characterised. It was suggested that these mAbs could be used in studying haemocyte differentiation, behaviour and function in P. monodon. In the present study, the reaction of these mAbs on P. monodon was compared with other crustaceans and disparate taxa. The mAbs also reacted with haemolymph components of three freshwater crustaceans, a terrestrial isopod crustacean and with coelomic fluid of an annelid. No reactions were observed with haemolymph of an insect and a mollusc, nor with blood cells of two vertebrates. This comparative study shows reactivity of the mAbs with a wide range of crustaceans and related animals and suggests that well conserved molecules are recognised, which may indicate functional importance. Well-described mAbs can be used in studies of the crustacean defence system and may finally result in a better insight into this system.