Antibody-coated bacteria were found in only two of 34 urine sediments from 19 catheterized patients infected with a single epidemic strain of Pseudomonas aeruginosa, whereas 12 of 19 urine sediments from 16 outpatients contained antibody-coated P. aeruginosa. In urine sediments, individual cells and microcolonies of the epidemic strain of P. aeruginosa were enclosed in ruthenium red (polysaccharide)-positive material. This strain was extremely mucoid when grown in a liquid medium for enhancement of mucoid formation. Renal infections was present in some patients, as determined by the bladder washout test and by titers of antibody in serum, and antibody was present in the urine but not coating P. aeruginosa. We conclude that the mucoid layer interfered with antibody coating of the epidemic strain of P. aeruginosa.