1. 1. A membrane fraction derived from deflagellated cells of Salmonella promotes polymerization of flagellin into “P-filaments” (ref. 1). The present study was undertaken to investigate whether or not changes in primary structure of flagellin are prerequisite to this polymerization. 2. 2. In this study we used the “soluble fraction” instead of “membrane fraction” for the initiation of polymerization 2. Isotope experiments showed that with addition of the soluble fraction more than 90% of the total flagellin polymerizes into P-filaments. 3. 3. Quantitative analysis of amino terminal residues showed that polymerization is not associated with cleavage of peptide bonds in flagellin. 4. 4. P-filament and intact flagellin were compared as to amino acid composition, amino and carboxyl terminal residues and tryptic peptide map; in these respects they were indistinguishable. 5. 5. From these experimental results, it was concluded that the primary structure of flagellin remains unchanged after polymerization into P-filament.