Mice were made tolerant to DNFB contact sensitization by pretreatment with DNB-SO3. Lymph node (LN) or spleen cells from such tolerant mice could specifically suppress the development of contact sensitivity to DNFB when adoptively transferred into normal (lightly irradiated) recipients. Mice receiving LN or spleen cells from tolerant donors were skin-painted with DNFB but failed to develop significant skin reactions when challenged with DNFB on the ears. LN cells from these same recipients also did not show antigen-driven DNA synthesis in vitro when stimulated with DNB-SO3. Tolerance was shown to be T cell dependent as prior treatment of tolerant LN cells with anti-ϑ serum and complement inhibited the adoptive transfer of tolerance while passage of these cells over an anti-immunoglobulin column had no such effect. Adoptive transfer of tolerance could not be accomplished via transfer of serum from tolerant donors. While lymphocytes from tolerant donors could prevent the development of contact sensitivity by normal recipients, they could not inhibit the expression of contact sensitivity of previously sensitized cells. Thus, cells from tolerant animals appear to inhibit the afferent limb but not the efferent limb of contact sensitivity. In this context, they may be termed “suppressor T cells.”