In this work, we studied the neuromuscular blockade caused by Micrurus altirostris venom (0.1-10 μg/mL) in indirect stimulated chick biventer cervicis and mouse phrenic nerve-diaphragm preparations and the ability of commercial antivenom (Instituto Butantan) and antiserum raised in rabbits to neutralize neurotoxicity and lethality in chicks and mice (LD50 0.042 and 0.255 mg/kg), injected i.m. and i.p., respectively, with venom (5 LD50):antivenom or antiserum mixtures (n = 6) of 1:1-1:2.5-1:5-1:10-1:20. The venom caused a complete and irreversible neuromuscular blockade in both preparations, inhibited the acetylcholine and carbachol contractures, without interfering on KCl response. The neuromuscular blockade was not Ca2+ or temperature-dependent and did not affect the response to direct stimulation. Only a venom:antivenom or antiserum ratio of 1:20 neutralized the neuromuscular blockade in vitro and protected chicks and mice against 5 LD50 of venom. 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