The in vitro production of IgA-anti-tetanus toxoid antibodies (IgA-Tet) by human peripheral blood lymphocytes (PBL) was assessed at various times after in vivo intramuscular tetanus toxoid immunization. Five days after immunization, T cell- and mitogen-independent synthesis of in vitro IgG-Tet, but not IgM-Tet or IgA-Tet, was detected. Two to six weeks after immunization, pokeweed mitogen (PWM)-stimulated cultures of B and T cells produced IgM-Tet and IgG-Tet but not IgA-Tet. We did, however, find that 75% of the serum samples of immunized individuals showed increases in IgA-Tet. Mitogen-dependent in vitro total IgA synthesis was detected in human PBL. Cells synthesizing the majority of PWM-induced IgA bear the phenotype Ig+ CR+ FcγR±. The majority of PPD-induced IgA synthesis was by cells lacking surface membrane IgD, similar to PPD-reactive IgG secretors. These results indicate that intramuscular immunization does not result in circulating B cells that can secrete antigen-specific IgA in mitogen-driven in vitro cultures. Moreover, multiple B-cell subsets are responsible for in vitro IgA production. © 1982 Plenum Publishing Corporation.