A cell-transfer system was employed in the present work to investigate several characteristics of the capacity of immune and normal lymphoid cells to transfer the delayed response to methylated human serum albumin in lethally irradiated syngeneic recipients. Spleen cells derived from donor mice immunized with goose erythrocytes were far less effective in transferring responsiveness when compared with equal numbers of normal cells. Statistical analyses indicated a frequency of 1 reactive cell or cell unit in 1.3 x 10(7) normal cells and in 6.2 x 10(7) immune cells. These findings provided confirmatory evidence that antigen-induced suppression (antigenic competition) employing sequential administration of two non-cross-reacting antigens is due to relative deficits of immunocompetent cells generated by lymphoproliferation in lymphoid tissues secondary to immunization with the initial antigen. The cellular deficit in the immune population was shown to be resident in a thymus cell population, which restored the number of responders to a level equivalent to the normal population. The thymic cell was akin to the antigen-reactive cell. The cell limiting the degree of response, that is the effector cell for both normal and immune cell populations, was of bone marrow origin. Both populations of cells were shown to act in synergy to reconstitute the delayed response to the antigen.