1. The complex flagella of Rhizobium lupini H13-3 differ from plain bacterial flagella in the fine structure of their filaments dominated by conspicuous helical bands, in their fragility and their resistance against heat decomposition. To elucidate the basis of these differences, the composition of complex filaments and their subunits was analysed. 2. Isolated complex flagella containing the filament and hook protions were purified by differential centrifugation. Hooks were separated by ultracentrifugation after acid degradation of filaments at pH 2. The complex filaments consist of 43 000 dalton monomers (cx-flagellin), the hooks are composed of 41 000 dalton subunits. 3. Amino acid analysis of cx-flagellin indicated the presence of approx. 417 amino acid residues. These comprise 47% hydrophobic residues and 21% Asp and Glu (or amides), but no Cys, His, Pro and Trp. No carbohydrate, phosphate or lipid moieties have been detected. Fingerprint analysis after tryptic digestion yields approx. 36 peptides, about half of them clustered in the neutral region. A comparison with the composition of varous known flagellins from plain flagella indicates a 7% higher content of hydrophobic amino acid residues in complex filaments; this is largely compensated for by the higher content of Glu and Asp (presumably as Gln and Asn) in plain filaments. 4. Immunodiffusion and immunoelectrophoresis of cx-flagellin yield single precipitin bands indicating homogeneity. In contrast, isoelectric focusing lead to three close-running bands around pH4.7. When isolated, the two major bands again produced an "isoelectric spectrum" suggesting that it reflects an allomorphism of cx-flagellin. 5. Self-assembly experiments with cx-flagellin lead to coiled fibres including helical regions, but not to intact filaments. The products resemble heat-denatured complex filaments and may represent intermediates between monomers and complete polymers.