Flagellar filaments of P. aeruginosa are isolated and purified by cultivating the cells in fluid medium, followed by shaking in a rotation-homogenizer and by fractional centrifugation. The electronmicroscopically pure preparations are partially disintegrated by heating at 60-100 degrees C/30 min. The flagellin repolymerizes spontaneously into flagella-like but straight filaments. In the indirect fluorescent antibody test, isolated flagellar filaments and polymerized flagellin-filaments with antiflagellar antibodies like flagella of native cells.