IL-4 signaling into a cell occurs via assembly of a receptor complex that consists of a high-affinity IL-4Rα chain and a low affinity chain, where the low-affinity chain is either γc or IL-13Rα1. It has been previously shown that mutational disruption of the low affinity interface in the IL-4DM (double mutein) yields an antagonist that inhibits IL-4 as well as IL-13-dependent responses. The present study reveals that new types of IL-4 antagonists can be generated by site-specific chemical modification. The chemically modified IL-4 analogues consist of (1) mixed disulfides created by refolding IL-4 cysteine muteins in the presence of different thiol compounds or (2) maleimide conjugates created by modifying cysteine muteins with maleimide derivatives. IL-4 analogues chemically modified at position 121 retain marginal binding affinity to γc or IL-13Rα1 receptor ectodomains during SPR interaction analysis. The biological activity of the analogues is strongly reduced in HEK-Blue IL-4/IL-13 cells as well as in Jurkat cells. Since the IL-4 analogues modified at position 121 have the ability to inhibit γc (IL-4)- and IL13Rα1 (IL-4/IL-13)-dependent responses in Jurkat and HEK-Blue cell lines, they effectively act as IL-4 antagonists. The results of our IL-4 study provide the first example of a cytokine that is transformed into a competitive inhibitor by site-specific chemical modification.