Type-3 fimbriae isolated from members of five different species of Klebsiella were 4-5 nm in diameter and agglutinated the tannic acid-treated erythrocytes of ox and, in some cases, the untanned erythrocytes of fowl. In sodium dodecyl sulphate-polyacrylamide gel electrophoresis, the type-3 fimbrial proteins had mol. wts in the range 19 500-21 500. Hydrophobic amino acids comprised 39.6% of all the amino acids of the type-3 fimbrial protein of K. oxytoca strain 70/1. The type-1 fimbrial protein of Klebsiella had a mol. wt of c. 18 000 and the type-1 fimbriae were serologically distinct from the type-3 fimbriae. Our results for the type-3 fimbriae of Klebsiella were compared with those of others for morphologically similar and serologically related thin fimbriae of Salmonella and Yersinia.