Various haemolymph components of the shrimp Penaeus japonicus were identified and characterised by monoclonal antibodies. Three groups of monoclonal antibodies were raised. Their reactivity to haemocyte types and/or secreted molecules was determined by immunofluorescence and the molecular masses of the antigens were analysed by western-blotting. A 170 kDa protein, in reducing conditions, was recognized by four panhaemocytic monoclonal antibodies from group 1. This protein was present both in the plasma and in the haemocytes from which it appears to be secreted. The shrimp haemocytes were separated by isopycnic centrifugation on a Percoll gradient and the different subpopulations were antigenically analysed using the two monoclonal antibodies, 40E2-2A and 40E10-2B, from group 2. The granular cells were labelled by 40E2-2A which was specific for a protein of 142 kDa also present in plasma. By comparison, the 40E10-2B monoclonal antibody was assumed to be the marker for small hyaline and semi-granular cells since the granular ones were not labelled. Moreover, the antigen immunoprecipitated by this monoclonal antibody was shown to have different molecular masses of 250, 150, 66 and 27 kDa under nonreducing conditions. It appeared to be secreted by the haemocytes. Some plasma proteins were recognized by the third group of monoclonal antibodies. The antibodies, designated 41D11-3A, 42C11-3B and 42E8-3C, all immunoprecipitated a protein with an apparent molecular mass of 180 kDa under reduced conditions. The 44E6-3D antibody was specific for a 75 kDa protein under reduced conditions and was shown to be immunoreactive against P. japonicus haemocyanin extract. An antigenic relationship was studied for the haemolymph components of P. japonicus and two other shrimp species, P. vannamei and P. indicus. Hypotheses about the possible nature of the proteins, antigenically identified, are advanced.