T. nattereri (niquim) is a venomous fish involved in many human accidents in Brazil. The clinical picture includes mild local erythema, severe edema, intense pain and rapid progression to necrosis. The present therapy with anti-inflammatory and analgesic drugs is ineffective and, therefore, we decided to assess serum therapy as an alternative treatment using an experimental antivenom. The antivenom used was raised in rabbits showing an ELISA antibody titer of 1:8,192,000 and its ability to neutralize lethality, necrosis, nociception and edema was evaluated both by pre-incubating the venom with antivenom before injection into mice or by independent injections of venom and antivenom. Lethality was completely neutralized by pre-incubation (ED(50)=141.5 microl/mg) while necrosis and nociception were neutralized by pre-incubation or the independent injection of antivenom. Edema was only partially prevented even when large amounts of antivenom were used. These data suggest that antivenom may be a promising treatment for patients stung by T. nattereri and suggest the viability of producing a horse antivenom for use in clinical trials.