Testing for urinary excretion of capsular polysaccharide antigen was carried out in 40 four-month-old Navajo infants who had received injections of aHaemophilus influenzaetype bNeisseria meningitidisouter membrane protein conjugate vaccine (PedvaxHIB®; Merck, Sharp and Dohme Research Laboratories) as part of an ongoing efficacy trial of the vaccine. Urine from 12 placebo recipients was also analyzed. Urine samples were collected on the day of injection (the first voided urine following the injection) and 3, 7, 10, 14, 21 and 30 days later. All vaccine recipients had at least 1 positive specimen. Vaccine recipients excreted antigen for a median period of 14 days after injection. On the first day 54% of vaccinees excreted antigen. Antigen was excreted by 89% of vaccinees on Day 3, 79% on Day 7, 82% on Day 10, 64% on Day 14, 56% on Day 21 and 41% on Day 30. Urine from placebo recipients tested positive in 12% on Day 1, 18% on Day 3, none on Day 7, 14% on Day 10, 11% on Day 14, 10% on Day 21 and none on Day 30. The rate of positive test results was significantly higher among vaccine recipients than among controls. Physicians should not rely on urinary antigen detection tests for predicting the presence of invasive disease caused byH. influenzaetype b in infants for at least 30 days after injections with this conjugate vaccine, and possibly longer.