A simple membrane immunobead assay (MIA) for detecting ciguatoxin (CTX) and related polyethers directly from fish tissue is presented. A membrane laminated onto a solid plastic support is immersed with a piece of fish tissue in methanol. The membrane is thoroughly dried and placed into an immunobead suspension containing polystyrene particles coated with monoclonal antibody to CTX (MAb-CTX). Two beads of different diameter and color are used. The color intensity of the membrane is related to the concentration of the toxin bound to the membrane. Twelve of 13 fish implicated in human ciguatera fish poisoning showed borderline or positive responses in the assay. A Sphyraena barracuda sample that tested negative with the MIA and was highly toxic with the mouse toxicity bioassay showed only weak CTX-like toxin activity in the guinea pig atrial assay, indicating that the major toxin in the sample was not CTX-like. Examination of 154 routinely caught reef fish from Hawaii, Kosrae, and Kwajalein by MIA found 132 (86%) negative and 8 (5%) positive for CTX, with 14 (9%) giving a borderline response. Fish from Hawaii showed a higher frequency of borderline or positive responses than those from Kosrae and Kwajalein, probably because several species of fish from several islands of Hawaii were tested, whereas only one species from a single area was examined from each of the islands of Kosrae and Kwajalein.