Delayed-type hypersensitivity (DTH) 1 develops without adjuvant if the dose of SRBC given is appropriate to the route of inoculation (1). Increasing the amount of antigen reduces and eventually abolishes all evidence of DTH. Animals blocked by an excessive dose of SRBC cannot be subsequently sensitized, even when an optimal dose of antigen is given subcutaneously, the most reliable route for inducing DTH in the mouse. DTH does develop, however, when a massive intravenous dose of antigen is given to splenectomized mice, or mice that have received cyclophosphamide before immunization (2). The suppression of DTH cannot therefore be due to excess antigen, as such. The serum of DTH-suppressed mice will partially inhibit the induction and expression of DTH in normal recipients, an effect which is substantially increased by partial absorption of the serum with specific antigen (3). In contrast to a high dose of antigen (4), absorbed serum does not inhibit the proliferative response or the formation of plaque-forming cells (PFC) in lymph nodes draining the site of antigen injection. Moreover, the specific antibody titer is increased by absorbed blocking serum even though the induction of DTH is totally suppressed by it.In light of indications that the products of the interaction between antigen and antibody depress the activated T cells which mediate DTH, and insofar as pretreatment with specific IGM antibody has been reported to enhance the antibody response (5, 6), an important question arises concerning the functional relationship between the mediators of DTH and the T cells responsible for helper activity in the antibody response. Obviously, a successful antibody response depends upon cooperation between the various cellular and humoral elements involved (7). It is possible that this can be accomplished efficiently only in certain sites, and that the local enactment of a DTH reaction is the process whereby the participating elements are assembled (8). After intravenous immunization, the spleen could be the locus of this coordinated series of events. Evidence in support of this concept is presented in the present report,