The crystal structure of the protein postsynaptic neurotoxin, erabutoxin b, has 
been refined at 0.140-nm resolution (R = 0.22) by restrained least-squares and 
interactive computer graphics. The study has established complete structural 
identity of the two sea-snake venom toxins, erabutoxin b and neurotoxin b, 
isolated from Laticauda semifasciata snakes taken in different Pacific Ocean 
waters. Two chemical-sequence inversion errors in erabutoxin b have been 
discovered during refinement, corrected and subsequently confirmed in both 
erabutoxin b and erabutoxin a by chemical analysis. The correct sequences are 
His6-Gln7, hitherto unsuspected, and Ser18-Pro19. The sequence correction 
His6-Gln7 explains the anomalous results of 1H NMR solution studies and those of 
early chemical modification experiments, which were in conflict with the 
previously published three-dimensional structure of erabutoxin b. On refinement, 
the five-stranded beta sheet described earlier is now shown to be discontinuous, 
split into a two-stranded beta loop and a three-stranded beta sheet. Unique 
features of the Pro44-Gly49 peripheral segment have now been identified. 51 
water molecule positions have been located.