A study of flagella and flagellin from six bacterial species led to the following generalizations: Flagella consist of proteins that, after removal from the cell bodies by shaking, can be isolated in highly purified condition by fractional centrifugation, or a combination of centrifugation and fractional precipitation with ammonium sulfate. Purification by centrifugation is aided by the removal of the “center-bottom” portion of precipitates, a fraction that often contains impurities. Experiments involving the stepwise saturation of the suspensions with ammonium sulfate indicated that the 0–0.1 and the 0.5–1.0 fractions are the least homogeneous. By discarding them one can obtain preparations of high purity. To isolate material of the “highest purity,” additional fractionation within the 0.1–0.5 saturation range is needed. Flagella dissociate at pH 2 apparently into individual flagellin molecules. In impure acid-dissociated preparations, a residue remains after centrifugation (the “pH 2 insoluble material”); this residue contains the bulk of the nonprotein substances in the preparation. Highly purified flagella contain no detectable “pH 2 insoluble material,” “hexose,” “pentose,” nucleic acid, or ash. While flagella precipitate over a wide range of ammonium sulfate concentrations, flagellin comes down within a very narrow range of saturation; in purified preparations there is no indication of contaminating proteins. The homogeneity of the preparations is also suggested by the demonstration that only one amino acid, alanine, is the N-terminal amino acid. On the assumption that each molecule of flagellin contains one terminal alanine, the minimum molecular weight is approximately 14,000–20,000, depending upon the organism. Similar values are obtained on the basis that each protein molecule contains one molecule of cysteine, and by ultracentrifugation. Flagellin of Proteus vulgaris was obtained in “crystalline” form. The amino acid composition of flagellin from Proteus vulgaris is given, and interesting features concerning the composition are mentioned. “Native” flagella and relatively unaltered flagellin preparations are not digested by trypsin, pepsin, and papain.