Summary Two hundred and eighty strains of Escherichia coli isolated from 85 female patients with chronic urinary tract infections in one hospital were tested for their serological and biochemical reactions. Biochemical and serological variations and degradation changes were observed in strains isolated repeatedly during the course of the disease in separate patients; their significance is discussed. From 34·2 per cent. of 38 patients the same sero- and bio-type of E. coli was repeatedly isolated, from 18·4 per cent. degraded strains probably the same as the original infecting type were isolated, and in 47·4 per cent. the sero-and bio-type changed during the course of the disease.