After immunization of six BALB/c mice with saxitoxin (STX) coupled to keyhole limpet hemocyanin only one mouse developed serum antibodies specific to STX. By hybridizing the splenocytes of this mouse with X63‐Ag8.653 myeloma cells one hybridoma line secreting antibodies to STX was produced. The antibody was designated 5C2 and proved to be IgM. In an indirect enzyme immunoassay using STX coupled to bovine serum albumin as the immobilized phase, the monoclonal antibody reacted with free STX in a concentration range of 4 to 10 μg per ml.