A competitive direct enzyme‐linked immunofiltration assay for the detection of saxitoxin was developed, using polyclonal antibodies against saxitoxin and a saxitoxin‐horseradish peroxidase conjugate. The test was performed in an eight‐well plastic test device, in which antibody‐coated nylon membranes were pressed tightly to an absorbent cellulose layer. Saxitoxin standard or sample extract solution, saxitoxin‐conjugate, and enzyme substrate/chromogen solution were sequentially added on to the membrane. The test was evaluated visually by comparing the intensity of the resulting coloured (blue) dot with that of a negative control. The detection limits for saxitoxin in buffer solution and in shellfish tissue were 4 ng/ml and 80 ng/g respectively, with an assay time of less than 15 min. Under the conditions of the immunofiltration assay, decarbamoyl‐saxitoxin, gonyautoxin 2/3, and neosaxitoxin standards (in buffer) gave a positive response at concentrations of about 10 ng/ml, 40 ng/ml, and 80 ng/ml, respectively. The relative cross‐reactivity of the antibody to these PSPs was similar when determined using both direct and indirect (using a saxitoxin—bovine serum albumin conjugate) competitive enzyme immunoassays in microtitre plate format. In competitive direct microtitre plate assays, the 50% binding values found for saxitoxin, decarbamoyl‐saxitoxin, gonyautoxin 2/3 and neosaxitoxin were 15 pg/ml, 47–5 pg/ml, 163–5 pg/ml, and 510 pg/ml respectively. In competitive indirect microtitre assay, the respective values were 138pg/ml, 404 pg/ml, 1582 pg/ml, and 6982 pg/ml.