Mytilus edulis develop progressive and fatal hemocytic neoplasia. We have generated monoclonal antibodies using neoplastic hemocytes from highly infected individuals collected from Puget Sound in Washington State (USA). Twenty-seven monoclonal antibodies against neoplatic cell antigens were obtained. Specificity of 2 purified antibodies, 14F1 and 16G10, was demonstrated by immunofluorescence when incubated with neoplastic hemocytes, whereas these anbidodies did not react with normal cells. This specificity was confirmed by immunogold labelling and electron microscopic visualization. The technique permitted localization of the epitopes recognized by the monoclonal antibodies on the plasma membrane. These monoclonal antibodies are suitable reagents for establishment of immunoassays for diagnosis and quantification of hemic neoplasia in the mussel.