Primary immunization of infants with protein-polysaccharide conjugate vaccines induces antipolysaccharide antibody and is highly effective in preventing invasive disease caused by encapsulated bacteria. However, recent experience from the UK indicates that this immunity is not sustained in the absence of booster doses of vaccine. This study aimed to establish the kinetics and phenotype of B-cell subpopulations responding to booster immunization with a heptavalent pneumococcal conjugate vaccine (Pnc7), which is to be introduced into the primary immunization schedule in the UK during 2006. Six adult volunteers received a booster dose of Pnc7 12-18 months after primary immunization. CD27hi CD38hi CD20(+/-) IgG antibody-forming cells were detected in peripheral blood with maximum frequency at days 6-7 after immunization. This was accompanied by a more prolonged rise in memory B cells that required in vitro stimulation with Staphylococcus aureus Cowan strain and interleukin-2 to induce antibody secretion. These data provide evidence for at least two subsets of antibody-forming cells involved in the secondary humoral response to a glycoconjugate vaccine in primed individuals. A briefly circulating subset of B cells that spontaneously secrete immunoglobulin G may be responsible for early defence against re-encountered encapsulated bacteria. However, the kinetics of the appearance of these cells may indicate that the humoral immune response is too slow in defence against an organism that invades within days of acquisition. The more sustained presence of a memory population may provide persistence of antipolysaccharide antibody after a booster dose of vaccine and may also include re-circulatory populations responsible for further anamnestic responses.