Bacterial flagellum consists of a basal body, a hook, HAP1 (hook-associated protein 1), HAP3, a long helical filament, and a cap (composed of HAP2), all connected in series. The mutant deficient in the HAP2 structural gene (fliD) of Salmonella typhimurium has flagella composed of only hook-HAP1-HAP3 and excretes flagellin monomers into the culture medium. However, when purified HAP2 was added to this mutant, the flagellin stopped leaking out and flagellar filaments grew. Turnover of HAP2 was not necessary for the growth of a filament. Therefore HAP2 facilitates the polymerization of endogenous flagellin, apparently without falling off the filament tip. This experimental system with exogenous HAP2 allowed us to synchronize filament growth; the average rate of filament growth can be estimated by measuring the length of grown filaments at various time periods in electron micrographs. The initial growth rate was about 30 nm/min, which corresponds to one flagellin per second.