Nasopharyngeal colonization of infant rats with Haemophilus influenzae type b was investigated by two methods of intranasal inoculation. After traumatic instillation of the bacteria, 100% of the animals became colonized, compared with 75.5% of animals after atraumatic instillation. Among colonized rats, significantly more animals in the traumatic group developed bacteremia compared with those in the atraumatic group. Rats in the traumatic group had an onset of bacteremia at a mean of 2.6 days after inoculation compared with 7.3 days in the atraumatic group. The duration of colonization and bacteremia was the same in both groups. The majority of heavily colonized rats developed bacteremia compared with none of the lightly colonized rats. Thus, the development of bacteremia appeared to be related independently to both heavy colonization and traumatic instillation. Protection against intraperitoneal bacterial challenge with H. influenzae type b developed in rats that had been bacteremic; in the majority of animals, this correlated with the development of serum bactericidal activity. Protection and bactericidal activity were only rarely observed in nonbacteremic rats that had been either heavily or lightly colonized. The development of serum bactericidal activity was not related to either the duration or peak level of bacteremia. Thus, in this rat model, H. influenzae type b nasopharyngeal colonization without bacteremia did not appear to stimulate circulating antibodies that protected the animals against challenge.