Strains of enteroaggregative E. scherichia colt (EAggEC), characterised by their pattern of adhesion to HEp-2 cells known as the `stacked brick' formation, are a significant cause of chronic diarrhoea in certain under-developed countries. Strains of EAggEC are detected either by a HEp-2 adhesion cell test or by an `aggregative adhesion' gene probe. The pathogenic mechanisms expressed by EAggEC are only poorly understood and the aim of the research described was to obtain a better understanding of how these bacteria cause disease. The adhesion of EAggEC to HEp-2 cells was shown in the majority of strains not to involve fimbriae and was thought to result from physical properties of strains such as charge, since EAggEC adhered to `fixed' HEp-2 cells and readily agglutinated a range of different erythrocytes. Certain strains of EAggEC, which also hybridised with a probe for diffuse adhesion, expressed membrane-associated proteins (MAPs) of 18 or 20 kDa responsible for HEp-2 adhesion. Divalent cations were essential for the expression of the MAPs, which did not contain disulphide bonds or have a quaternary structure. Strains of EAggEC did not express recognised subunit toxins such as Verocytotoxin or E. coil heat-labile toxin, and strains which hybridised with probes for enteroaggregativeh eat-stable toxin-1 did not produce E. coil heat-stable toxin detected by the infant mouse test. Some EAggEC strains (15%) had haemolytic properties. Certain strains expressed type II capsular polysaccharides and approximately 50% of strains expressed an aerobactin-mediated iron uptake system. It was concluded that strains of EAggEC belonged to a very diverse range of serotypes, and it was thought that this heterogeneity resulted from strains of E. coil readily acquiring the genes encoding the EAggEC phenotype. Strains of EAggEC were not associated with a single pathogenic phenotype and the ability of these bacteria to adhere to HEp-2 cells in a `stacked brick'-pattern remains the only common characteristic.