Despite the widespread use of antibiotics urinary tract infections (UTI) remain among the most common bacterial infections in the human population. About 5% of adult females are likely to have a UTI at any moment in time, while more than 50% of all women will experience a UTI at some stage in their lives [l]. Because individual women can alternate between symptomatic and asymptomatic episodes of infection it seems likely that the bacteria associated with overt infections are more pathogenic than their covert counterparts. The observation that most cases of UTI are caused by strains of Escherichia coli with a restricted range of 0-serotypes [2-41, and that the pattern of these 0-serotypes does not correspond to the distribution for Esch. coli strains in the normal faecal flora [S], supports the idea that urinary pathogens possess definite determinants of virulence. Furthermore, carriage of genes coding for haemolysin synthesis [3, 61, resistance to the cidal effect of serum [7] and the ability to cause mannoseresistant haemagglutination (MRHA) of human erythrocytes [3, 61 are all associated with the 0-serotypes which predominate in urinary Esch. coli isolates. Such markers of virulence appear to be particularly common among bacterial strains isolated from patients with infections of the upper urinary tract [2], and in particular, the MRHA adhesions designated P fimbriae have been shown to occur with a frequency of between 81% [8] and 94% [9] in subcultured pyelonephritic strains of Esch. coli. A great deal of attention has been paid in the last decade to the identification and charac-