The paralytic poison produced by the dinoflagellate Gonyaulax catenella in axenic culture has been isolated in pure form. A study of its chemical, physical, and biological properties establishes that it is identical in chemical structure to saxitoxin, the poison isolated from toxic Alaska butter clams, and to the poison isolated from toxic California sea mussels; Reverse thin film dialysis and gel filtration have been used to study the interaction and binding of adrenocorticotropin (ACTH) with proteins. Comparisons have been made with a tricosapeptide analog. A high degree of interaction is shown to occur between both of these peptides and proteins. Albumin is particularly effective in binding the hormone peptides, and this binding has been found to be concentration dependent. Consistent and striking differences are observed between the bound and unbound forms of these peptides with regard to their hormonal behavior. From these results, and a knowledge of the portions of the peptide chains which are required for activity, some conclusions are drawn regarding the amino acid sequences which may be primarily involved in binding. The results indicate the probable major site of binding of the tricosapeptide to be at amino acid residues 15-18. It is also shown that amino acid residues 24-39 have an important effect on the hormone-protein interaction