The horseshoe crab, Carcinoscorpius rotundicauda, when injected intracardially with tetrodotoxin (TTX) at a dosage of 10 x LD50 of mice showed no mortality. The resulting haemolymph extracted 24 hr later from these crabs caused no fatalities when injected into mice. TLC analysis revealed that cell free haemolymph (CFH) had converted a single yellow fluorescent spot of TTX into two blue fluorescent components when the reaction was carried out at 25 degrees C. However, no fluorescence was observed when TTX was reacted with CFH at 37 degrees C. There was also an increase in the intensity of protein bands on SDS-PAGE gel corresponding to 26,000 and 28,000 mol. wts. This may be the result of the TTX-neutralization reaction in CFH. The pretreatment of TTX with CFH reduced the toxicity of TTX in mice by two-fold. The anti-TTX activity is temperature and time dependent as well as heat stable. Protein denaturation of the haemolymph suggests that proteinaceous factor(s) may also play a role in TTX-neutralizing activity of CFH. It is therefore possible that the detoxification system is attributed to concerted actions of both proteinaceous and non-proteinaceous factors in the haemolymph.