Nodularin (Nod) is a cyclic pentapeptide hepatotoxin produced by the cyanobacterial genus Nodularia living in brackish waters and coastal lagoons. The toxicity of Nod is due to specific inhibition of the type-1 and type-2A intracellular protein phosphatases (PP1 and PP2A, respectively). We have developed a monoclonal antibody against Nod using chemical modification (aminoethylation) of one of its core amino acids, N-methyldehydrobutyrine. The developed antibody is highly specific for Nod, with negligible reactivity to the closely related cyanobacterial toxin microcystin (MC). The monoclonal antibody was employed for quantitative competitive ELISA assay. The analytical sensitivity of the assay was up to 0.2 ng/ml. Comparison of the developed ELISA test with HPLC-based measurements of Nod, with both laboratory and field samples, showed a good correspondence between the results yielded by these two methods. The antibodies developed by this technique provide means for developing extremely sensitive and specific analytical assays for direct measurement of nodularin and related toxins in cyanobacterial or water samples.