A monoclonal antibody against tetrodotoxin (TTX) was produced from the hybridoma cell line T6D9, which was established by the fusion of Sp2/0 myeloma cells with spleen cells isolated from a Balb/c mouse immunized with the TTX-keyhole limpet hemocyanin (KLH) conjugate. This monoclonal antibody belongs to the IgG1 subclass; the affinity constant of the antibody is 2.4 × 10(-8) mol l(-1). The relative cross-reactivity of the antibody with TTX was 100%, but with saxitoxin, KLH and bovine serum albumin (BSA) it was less than 1%, respectively. The titre of the antibody in ascites was 6.4 × 10(6); the reference working concentration was 1:1.2 × 10(5). By using this monoclonal antibody, a competitive indirect enzyme-linked immunoabsorbant assay (ELISA) for the analysis of TTX was developed. The linear portion of the dose-response curve of TTX concentration was in range 5-500 ng ml(-1). The limit of detection was 5 ng ml(-1) according 10% inhibition with TTX to anti-TTX monoclonal antibody. The concentration of TTX inhibiting 50% of antibody binding was about 50 ng ml(-1). The recoveries from TTX spiked samples were 79.5-109.5%. In addition, the toxicity of some wild puffer fish specimens captured from south-east China and the Yangzi River in Jiangsu province was determined. The results indicate that the toxicity and toxin tissue distribution vary in different species of wild puffer fish.