L. Levine, H. Fujiki, H. B. Gjika and H. Van Vunakis. A radioimmunoassay for palytoxin. Toxicon 26, 1115–1121, 1988.—Palytoxin, labelled with 125I-Bolton-Hunter reagent on its terminal amino group, bound specifically to rabbit anti-palytoxin. The extent of binding increased progressively with repeated immunizations. After absorption of the rabbit IgGs with a goat anti-rabbit IgG, binding was reduced greater than 95%. For 50% inhibition of binding in the 125I-palytoxin-anti-palytoxin reaction 0.27 pmoles of unlabelled palytoxin was required. Maitotoxin, teleocidin, okadaic acid, debromoaplysiatoxin and 12-O-tetradecanoylphorbol-13-acetate, when tested at 10–100-fold higher concentrations than palytoxin did not affect binding. Palytoxin's serologic activity was stable after 60 min exposure to 100°C and after 60 min exposure to 0.1 N HCl at 50°C, but its capacity to stimulate the arachidonic acid metabolism of rat liver cells was reduced after the 60 min exposure to 0.1 N HCl treatments at 35°C or 0.01 N HCl at 50°C. The average binding constant ( K 0) as determined by separation of antibody-bound palytoxin from free palytoxin by the double antibody technique was 4.9 × 10 9 M −1 at 0°C. This apparent average association constant increased with increasing temperature suggesting that palytoxin's epitope, most likely hydrophilic, is bound to H 2O and the H 2O is displaced before binding to the antibody's paratope.