The kinetics of bacteremia and capsular antigenemia in infant rats infected with Haemophilus influenzae type b were measured by quantitative bacterial counts in blood and counterimmunoelectrophoresis of plasma. After intraperitoneal inoculation with 10(4) colony-forming units (cfu) of H. influenzae type b, bacteremia was detected in 100% of animals at 12 hr after inoculation (mean, 16,500 cfu/ml) and by two days exceeded 10(5) cfu/ml in most animals. Despite these high levels of bacteremia, capsular antigen was detected infrequently during the early phase of experimental infection; it was present in 20% of animals at 12 hr and in 50% at one day. Peak levels of antigen in blood occurred two to three days after inoculation and coincided with the histologic appearance of meningitis. Thereafter, the frequency of antigenemia declined and paralleled the decline in quantitative bacterial counts in blood. Since detection of antigen was dependent on the occurrence of prolonged infection, counterimmunoelectrophoresis proved to be an insensitive method for early diagnosis.