Adhesions of 211 strains of uropathogenic Escherichia coli and 19 strains of normal fecal E. coli were characterized by patterns of agglutination with human erythrocytes, Saccharomyces cerevisiae, and horse erythrocytes coated with the P blood-group receptor (P). Mannose-resistant (MR) hemagglutination was significantly associated with P agglutination (P less than .001). E. coli expressing MR and/or P (MR/P) agglutinins concurrently with mannose-sensitive (MS) agglutinins predominated in all clinical categories. The highest percentage of E. coli demonstrating MR/P agglutinins, in the absence of MS agglutinins, was recovered from patients with acute pyelonephritis (35%) compared with percentages of patients with chronic pyelonephritis (13%), asymptomatic bacteriuria (16%), cystitis (11%), and normal fecal control E. coli (11%). Sixty-nine percent of E. coli isolates causing acute pyelonephritis agglutinated P-coated horse erythrocytes compared with only 11% of the fecal isolates. Strains expressing MR/P agglutinins (in the absence of MS agglutinins) isolated from patients with acute pyelonephritis, chronic pyelonephritis, and asymptomatic bacteriuria were significantly associated with the presence of antibody-coated bacteria in patients' urine sediments (P less than .010), an observation indicative of an immune response associated with bacterial invasion of host tissues.