Carbon blockade of mice did not affect cytotoxic antibody production after immunization with allogeneic cells, nor did it affect the immunosuppressive activity of alloantibody. Though it inhibited the alloantibody-mediated increase in hepatic localization of allogeneic lymphoid cells it had no effect on the normal hepatic localization of allogeneic lymphoid cells. Alloantibody administered before allogeneic cells or 6 h after was equally immunosuppressive, but anti-lymphocyte antibody only had potent immunosuppressive activity when given just before the antigenic cells. These results suggest than an afferent mechanism of suppression of the humoral antibody response can be exerted by anti-lymphocyte serum and that alloantibody may also act by this method when given before the antigen. As alloantibody is also highly potent when given after the antigen when an afferent mechanism is unlikely to occur, studies on the specificity of immunosuppression, particularly the likely requirement for antibody to I region determinants, would best be conducted by giving the antibody after the antigen, thereby avoiding any possible afferent inhibitory activity.