1. The toxic principles in the venom of the sea-snake Laticauda semifasciata 
were separated into two components by CM-cellulose chromatography and obtained 
in crystalline forms. They were named ;erabutoxins a and b'. 2. The homogeneity 
of each toxin was shown by rechromatography, by disk electrophoresis, by 
ultracentrifuging, by toxicity measurements before and after repeated 
crystallizations and by N-terminal analysis. 3. They had molecular weights of 
about 7000. Both of them contained 61 (or 62) amino acid residues/molecule. The 
only difference between erabutoxins a and b was that one of the aspartic acid 
(or asparagine) residues in erabutoxin a was replaced by a histidine residue in 
erabutoxin b. 4. Both of the toxins had LD(50) values of 0.15mug./g. body wt. 
for mice and 0.07mug./g. for rats. It was shown with frog-muscle preparations 
that they acted on postsynaptic membrane to block neuromuscular transmission.