In vivo immunization with tetanus toxoid causes the appearance of B cells in the circulation that can produce IgG-antitetanus toxoid antibody in vitro after stimulation by T cells and pokeweed mitogen. Addition of soluble tetanus toxoid to these cultures, however, causes a profound inhibition of the in vitro IgG, but not IgM, antitetanus toxoid antibody production. Treatment of the lymphocytes with 10 microgram of tetanus toxoid for as little as 1 hr at 37 degrees C was sufficient to promote this antigen-specific inhibition. The antigen could be added, however, as late as 3 days after the culture initiation for the inhibition to be observed. Cell separation studies indicated that the inhibitory activity could be found in both the E-rosette positive and E-rosette negative populations. Further studies on the non-E-rosetting cells showed that: 1) inhibition induced in antigen-treated cells could not be transferred to untreated cells; 2) adherent cells were not responsible for the inhibition; 3) treatment of the surface Ig+ cells with antigen was sufficient for the inhibitory effects; and 4) in vivo generated lymphoblastoid cells that produce IgG-antitetanus toxoid antibody in vitro but are surface Ig- were not inhibited by soluble antigen. The results in this study thus indicate that cells in both the E-rosette positive and negative subsets can be responsible for the antigen-induced inhibition of in vitro antibody production observed in the majority of immunized donors.