It was found that K99 pili from enterotoxigenic Escherichia coli (of bovine origin) could be extracted by treatment with 3M-KSCN solution. The K99 pili were purified by preparative isoelectric focusing to apparent homogeneity as judged by the presence of a single band on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis; the molecular weight of this component was calculated to be 12 600 +/- 300. This indicated that the K99 pili were composed of a single subunit. On analytical ultracentrifugation, a single boundary with an s20,w of 12.2 S at a concentration of 0.42 mg/ml was observed. The average length of purified pili at zero concentration was approx. 160 nm and the diameter was 7.4 +/- 0.6 nm. Amino acid analysis of the purified K99 pili revealed that sulphur-containing amino acids, cysteine and methionine, were absent. Aromatic amino acids, phenylalanine and tyrosine, previously reported to be absent [Isaacson (1977) Infect. Immun. 15. 272-279], constituted 7.14% of the total amino acid residues present. On immunoelectrophoresis, purified K99 pili migrated towards the cathode and caused mannose-resistant haemagglutination of horse, but not of sheep or guinea-pig, red blood cells. Pili from enterotoxigenic E. coli of porcine and human origin and from another bacterial species, namely Fusiformis nodosus, could also be extracted by the treatment of respective micro-organisms with 3 M-KSCN.