The kinetics of the popliteal lymph node response to an allograft of lymphoid cells injected into the foot pad were examined after challenge with strong (Ag-B) and weak (non Ag-B) transplantation antigens. The response to weak antigens was characterised by a lag period of 2 days. The response to strong antigens was more rapid in onset. Active immunisation against weak antigens accelerated the response so that it came to resemble the immediate response to strong antigens. Active immunisation against strong antigens depressed the response. This depression was reproduced by giving passive antiserum to normal recipients. Passive antiserum produced by immunisation against weak antigens was also suppressive. Adoptive transfer of immune lymphoid cells reproduced the effects seen with passive transfer of antiserum. These findings indicate that clonal expansion of antigen-sensitive cells is characteristic of immunity to weak and not to strong antigens and that antibody is produced in both situations and has a similar suppressive effect on the cell-mediated response in addition to its direct effect on the graft.