Bothropstoxin-I (BthTX-I), the principal myotoxin of Bothrops jararacussu venom, is devoid of phospholipase A 2 (PLA 2 ) activity but capable of blocking neuromuscular transmission in mouse nerve-muscle preparations. In this study, the ability of crotoxin antiserum and heparin in preventing the neurotoxic and myotoxic effects of BthTX-I was investigated. Phrenic nerve-diaphragm preparations (PND) stimulated indirectly with supramaximal stimuli (0.2ms, 0.1Hz) were incubated with BthTX-I (20μg/ml) alone or with BthTX-I preincubated with antiserum or heparin for 30min at 37 o C prior to testing. Control preparations were incubated with Tyrode solution, antiserum or heparin alone. BthTX-I (20μg/ml) produced 50% neuromuscular blockade in the PND preparations in 31+/-4min, with complete blockade occurring in 120min. The antiserum and heparin significantly prevented the neuromuscular blockade caused by BthTX-I (84+/-4% and 100% protection, respectively). Light microscopy examination of the muscles at the end of the 120min incubation showed that BthTX-I damaged 48+/-6% of the fibers. Preincubating the toxin with antivenom significantly reduced the extent of this damage (only 15+/-4% of fibers affected, corresponding to 69% protection, P<0.01) whereas heparin offered no protection (34+/-7% of fibers affected, not significantly different from that seen with toxin alone). These results show that the antivenom was more effective in neutralizing the myotoxic effects of BthTX-I than was heparin.