HLA class II molecules play pivotal roles in antigen presentation to CD4 + T cells. We investigated signaling via HLA-DR molecules expressed on CD4 + T cells. When HLA-DR or CD3 molecules on cloned CD4 + T cells were cross-linked by solid-phase mAbs, T cells proliferated, and this resulted in anergy. Whereas cross-linking of HLA-DR and CD3 resulted in secretion of the same levels of IFN-γ and IL-8, secretion of IL-10 induced by cross-linking of HLA-DR was less than that induced by cross-linking of CD3 on CD4 + T cells. Interestingly, expression of p27 K i p 1 but not p21 C i p 1 increased after stimulation by either anti-HLA-DR or anti-CD3 mAb. This was indeed the case, when T cells were rendered anergic using a soluble form of antigenic peptide. In contrast, T cells stimulated by peptide-pulsed PBMC expressed little p27 K i p 1 . We propose that signaling via HLA-DR molecules on CD4 + T cells at least in part contributes to the induction of T cell anergy, through the upregulated expression of the p27 K i p 1 . The implication of our finding is that HLA-DR molecules play a role in human T cell anergy induced by a soluble form of antigenic peptide.