Pathogenic bacteria colonise their host animals by means of a complex set of interactions. The host defensive mechanisms attack foreign microorganisms and attempt to rid the animal of the invaders, while the bacteria express a variety of functions to ensure their survival under adverse conditions, some of which damage the host and cause the clinical symptoms of disease. Adhesins are the bacterial structures which mediate adherence to specific host tissues and therefore permit the colonisation of areas from which the bacteria would normally be removed. T4. I have genetically cloned and analysed an adhesin from a pathogenic strain of Escherichia coli isolated from a child with enteritis. The genetic information was transferred to laboratory strains of E. coli and was expressed under similar conditions as in the parent strain, generating material with the same adherence and antigenic properties. Thus, the cloned genes enabled laboratory strains to adhere to human colon, but not to duodenum, in the same manner as the parent. This probably accounts in large part for the tissue specificity of the pathogen which caused dysentery-like symptoms consistent with colonisation of, and damage to, the colon. The cloned genes encoded the synthesis of the adhesin as fine fibrils ('fimbriae') on the bacterial surface, approximately 2 nm in diameter. The 14,000 dalton protein subunits were assembled into very high molecular weight aggregates and were purified by size fractionation. The genetic determinant occupied about 6,000 basepairs of DNA, indicating a system of genes for the synthesis, export and assembly of functional adhesin. The genetic map was very similar to those of adhesins from another enteritis isolate and a urinary tract pathogen, suggesting an evolutionary relationship between these E. coli strains. However, the protein subunits of the three adhesins appear to differ, indicating a degree of divergence.