The mannose-specific adherence to yeast cells of Escherichia coli excreted in the urine of patients with urinary tract infections was compared with that of isolates from the same urine after growth of the bacteria in broth. The results revealed that although E. coli excreted in only 2 of 24 urine specimens exhibited mannose-specific adherence, about half of the broth cultures from these specimens did so. Examination of representative specimens of E. coli excreted in urine showed that coating antibodies, mannose-containing glycoproteins, and encapsulation were not responsible for the lack of the mannose-specific adherence. Our results suggest that E. coli strains that are genetically capable of exhibiting mannose-specific adherence may, when growing in the bladder, be in a phase of growth which suppresses the phenotypic expression of this trait. Mannose-specific adherence is indicative of the presence on the bacterial surface of adhesions (lectins) that bind the organisms to mannose residues on both epithelial and phagocytic cells. We propose that whereas at the initial stages of infection the bacteria may benefit from their ability to bind to mannose residues on epithelial cells, loss of this ability at the later stages of the infection is also beneficial, since the bacteria can no longer adhere to mannose residues on phagocytes, and are thus resistant to nonimmune phagocytosis.