Escherichia coli is the most common cause of infection of the urinary tract, particularly when obstruction to the free flow of urine is absent.1Investigation of this group of organisms has been simplified and rationalized by the studies of Kauffmann2who proposed a serological classification based on the presence in these bacteria of heatstable O antigens and of heat-labile K antigens. This technique has rarely been applied to the investigation of human illness because suitable specific sera have not been available. Work in this field has concerned itself largely with the identification of possible enteropathogenic E. coli. The exceedingly important problems surrounding infection of the urinary tract have not been attacked with the assistance of this potentially valuable tool. Plans were made recently for a long-term comprehensive study of urinary tract infection. Coincidentally, sera suitable for the identification of O groups of E. coli became available through the