The purpose of this study was to investigate whether preexisting immunity to components of a polysaccharide-protein conjugate influences the B-lymphocyte response to vaccination with the conjugate. Thirty-two healthy adults were vaccinated once or twice with a conjugate (PRP-D) consisting of Haemophilus influenzae type b capsular polysaccharide (PRP) and diphtheria toxoid (DT), and the response was related to the prevaccination levels of PRP and DT antibodies. Positive correlations were found between increases in plasma PRP (median, 32.0 micrograms/ml) and DT (1.14 IU/ml) antibodies and numbers of circulating PRP and DT antibody-secreting cells (AbSC) (postvaccination days 6 to 9). The B-cell responses (antibody response and AbSC) to both PRP and DT correlated positively with prevaccination levels of anti-DT. DT AbSC appeared earlier (peak, day 7) than PRP AbSC (peak, day 8). Individuals whose PRP AbSC peaked early (day 7) had higher prevaccination anti-DT levels than those who peaked later (P less than 0.05). In contrast, the prevaccination levels of anti-PRP did not correlate significantly with the magnitude of the antibody or AbSC response and did not affect the kinetics of the AbSC. Following revaccination with PRP-D, small increases in the level of PRP antibodies (median, 2.9 micrograms/ml; n = 11) were found; no significant increase in the level of DT antibodies was seen. The numbers of PRP AbSC were lower (P = 0.04) and peaked earlier (day 7) than after the first vaccination. The isotype pattern of PRP AbSC, which was dominated by immunoglobulin A (IgA) after the first vaccination, now showed a more equal distribution between IgG and IgA AbSC. It is concluded that after immunization with PRP-D both the magnitude and the kinetics of the antipolysaccharide B-cell response are influenced by prevaccination immunity to the carrier molecule.