SUMMARY: A method is described for assaying flagella of Salmonella typhimurium based on the incorporation of a 14C-labelled amino acid, mechanical separation of the flagella from the bacteria, and measurement of the radioactivity of the isolated flagella. There is no detectable lag before the appearance of L-[(G-3H]leucine, or L-[Me-14C]methionine in the flagella, and the rates of incorporation of these amino acids into flagella paralleled the rates of incorporation into the general cell protein. Mechanical removal of the flagella did not affect their subsequent rate of formation. The amino acid, ɛ-N-methyllysine, present in flagellin, can be labelled with either L-[U-14C]lysine or L-[Me-14C]methionine. Using ɛ-N-methyl-lysine as a marker, it has not been possible to detect soluble protein precursors in the formation of flagella by S. typhimurium.