An absolute requirement for monocytes was demonstrated in the T-cell proliferative response to tetanus toxoid (TT) antigen. Antigen-pulsed monocytes were shown to be effective in triggering T-cell proliferation. Using 125I-radiolabeled TT antigen, uptake by monocytes increased progressively over an 18-hr period, at which time 80-85% of the monocytes contained radiolabeled material. The ability of antigen-pulsed monocytes to trigger T-cell proliferation paralleled antigen uptake over an 18-hr period. Monocytes pulsed with antigen, then washed, lost their ability to trigger T-cell proliferation following a 24- to 48-hr culture period. Metabolic inhibitors blocked antigen uptake by monocytes and monocyte triggering of T-cell proliferation. Trypsin treatment of TT-pulsed monocytes did not affect the amount of antigen associated with monocytes or T-cell triggering by monocytes. Anti HLA-DR alloantibodies, which when added during antigen pulsing of monocytes inhibit the capacity of these monocytes to trigger T-cell proliferation, did not interfere with antigen uptake. These results indicate that human monocytes present antigen to T cells via an active process and in association with Dr determinants, and that the immunogenic moiety of antigen does not remain indefinitely available to the T cell.