The in vivo human humoral response to diphtheria toxoid-tetanus toxoid booster immunization was studied by isoelectric focusing analysis of sera obtained after immunization. The anti-diphtheria toxoid (immunoglobulin G [IgG]-Dip), anti-fragment A (IgG-Frag A), and anti-tetanus toxoid antibodies from 20 donors post-booster immunization were focused by using agarose isoelectric focusing and visualized by development with radiolabeled antigens. The quantities of the IgG-Dip and IgG-Frag A antibodies correlated with the number of bands seen on the isoelectric focusing pattern in that more bands were found in the spectrotypes of donors with high serum levels of antibody. No difference was apparent in the antibody spectrotypes obtained from sera of donors at successive times post-booster immunization. Individual heterogeneity of the different donors' spectrotypes was often found for IgG-Frag A antibodies, but a close comparison of several different donors revealed antibodies with the same spectrotype patterns. Thus, individual clones of antibody were revealed in humans after in vivo immunization, particularly when antibodies against antigens of restricted epitope size were analyzed. Additionally, the sharing of certain antibody spectrotypes among several individuals raised the possibility that certain antibody clones may be preferentially expressed in the human population.