Abstract: Disclosed herein are stable peptide antagonists based on the consensus yc-subunit binding site to inhibit the activity of yc-cytokines. Such peptide antagonists are capable of inhibiting the activity of multiple yc-cytokine family members. The yc-family cytokines are associated with important human diseases, such as leukemia, autoimmune diseases, collagen diseases, diabetes mellitus, skin diseases, degenerative neuronal diseases and graft-versus-host disease (GvHD). Thus, inhibitors of yc-cytokine activity are valuable therapeutic and cosmetic agents as well as research tools.