To characterize type B botulinum neurotoxin based on reliable data on the amino acid composition, three batches of the neurotoxin were analyzed. Each batch was isolated from a separate neurotoxin producing bacterial culture (strain Okra). Two batches were purified by the same method and one was purified by a different method. The toxin preparations were comparable in purity (judged by polyacrylamide gel--sodium dodecyl sulfate electrophoresis) and similar in amino acid composition. The best estimate of the number of amino acid residues per toxin molecule (mol. wt 152,000) was: Asp212,Thr54,Ser83,Glu130,Pro46,Gly61++ +,Ala44,Val54,CyS11,Met23,Ile144,Leu107 , Tyr81,Phe77,Lys118,His7,Arg39,Trp18.