The passage of spleen cells through a tightly packed nylon wool column partially separated two populations of cells required for the antigen-induced secondary proliferative response of T cells. The proliferating T cells were in the nylon wool column-passed cell population, but the addition of a small number of nylon-wool-adherent cells was required for their maximal proliferative response. Such adherent cells were non-T, non-B, Ia antigen-positive cells serving as antigen-presenting cells. Ia determinants on this cell type are encoded in two separate I subregions, I-A and I-E/C, and they are expressed simultaneously on the same cells. Antisera specific for I-J subregion gene products having as a known activity the killing of antigen-specific suppressor and helper T cells were unable to kill antigen-presenting adherent cells. The results indicated that at least I-J subregion gene products expressed on suppressor and helper T cells are not present on antigen-presenting accessory cells. The Ia specificities detected on accessory cells follow the cross-reactive pattern found in Ia molecules of B cells, and thus antigen-presenting cells carry Ia antigen identical to those of B cells, and not those of T cells.