The primary structure of Cerebratulus lacteus toxin B-II has been investigated by automated Edman degradation of the reduced, carboxymethylated protein and of tryptic peptides derived from the maleylated protein. As a result of these studies, the identity of all 55 amino acid residues in the polypeptide chain has been unambiguously determined. Hydroxyproline, an amino acid not normally found in nonfibrous proteins, occupies position 10 of toxin B-II. The sequence analysis of B-II led us to re-examine the sequence of Cerebratulus toxin B-IV. The revised structure of toxin B-IV is presented herein. Like B-II, toxin B-IV contains hydroxyproline at position 10 of its sequence. Comparison of the sequences of toxins B-II and B-IV reveals a high degree of homology between these two polypeptides, particularly within the NH2-terminal two-thirds of each chain.