AbstractThis study presents data from the crossreactivity analysis of purified ciguatoxin (CTX), okadaic acid (OA), and the East sphere or Fragment B‐C of OA with their homologous antibodies, monoclonal antibodies to ciguatoxin (MAb‐CTX) and okadaic acid (MAb‐OA). The test system used was the stick enzyme immunoassay. MAb‐CTX gave peak titers of 1.5ng, 10ng and 50ng respectively for CTX, East sphere and OA. Competitive inhibition analysis showed that 4ng purified CTX blocked completely MAb‐CTX reaction with crude CTX, OA and East sphere of OA blocked at similar concentrations (approximately 50ng). The activity with MAb‐OA in the homologous system with OA and East sphere was insignificant. This may be attributable to the improper concentrations used. The cross‐reactivity between MAb‐CTX with OA and its Fragment B‐C may cause difficulty in the test system in its application to assess toxic fish