The appearance of circulating B lymphocyte subpopulations responsible for the in vitro synthesis of IgG or IgM antitetanus toxoid antibody after in vivo booster immunization has been analyzed. Before booster immunization, only background levels of IgG antitetanus toxoid antibody were synthesized by B lymphocytes alone, or in combination with T cells and pokeweed mitogen, as measured by quantitative radioimmunoassay. Within 5 days after booster immunization, a population of B lymphocytes appeared within the circulation that had the capacity to synthesize IgG antitetanus toxoid antibody in the absence of T lymphocytes or pokeweed mitogen. These cells could be detected in the circulation until 10 to 12 days after immunization, and the antibody synthesized was inhibited by the inclusion of 50 μg/kl cyclohexamide in the culture medium. At the peak of this response, between 10 and 60% of the total IgG synthesized was IgG directed against tetanus toxoid. The cells responsible had a precursor frequency of up to 10-3 in the non-E-rosetting cell fraction obtained from peripheral blood and sedimented at 1 x G with volocities ranging from 4 to 11 mm/hr. The synthesis of IgM antitetanus toxoid antibody by this cell fraction was not detected. In contrast to the above B cells, lymphocytes capable of responding to pokeweed mitogen and T lymphocytes with the synthesis of IgG antitetanus toxoid anitbody did not appear in the circulation until 10 days after immunization. Limiting dilution analysis of the pokeweed mitogen responsive B lymphocytes 2 to 4 weeks after booster immunization gave a precursor frequency of 1 x 10-4. In the presence of T cells and PWM, the synthesis of IgM anti-Tet remained at constant levels before and for 3 weeks post-booster immunization. These results indicate that multiple subsets of circulating B cells can be functionally defined in humans.