Suppressor cells obtained from spleens of normal A mice, or factor extracted from these suppressor cells, abolished the syngeneic delayed-type hypersensitivity (syn-DTH) response of X-irradiated A mice injected with trinitrophenylated spleen cells and challenged with syngeneic lymphoblasts. Some of the physical, chemical and biological properties of the suppressive factor (SF) were characterized. The SF was relatively temperature-stable and its activity was destroyed by pronase (but not with RNase or DNase). The activity of the SF was absorbed on concanavalin A and anti-I-Jk Sepharose columns, suggesting that the factor is a glycoprotein-bearing I-Jk product. The approximate molecular weight of the factor is 50,000-60,000. The SF was absorbed on plastic adherent cells (but not on non-adherent cells). Adherent cells that absorbed the SF abrogated the ability of primed T cells to transfer the syn-DTH to naive X-irradiated recipients. In contrast, SF that was presented directly to the primed T cells failed to abolish their ability to transfer DTH. These findings suggest that the adherent cells serve as mediators, transferring the SF from factor-producing cells (Lyt-1+2+3+, I-Jk+ T cells) to target cells (Lyt-1+ primed T cells).