A monomeric form of human interleukin 10 (IL-10M1) has been engineered for detailed structure-function studies on IL-10 and its receptor complexes. Wild type IL-10 (wtIL-10) is a domain swapped dimer whose structural integrity depends on the intertwining of two peptide chains, wtIL-10 was converted to a monomeric isomer by inserting 6 amino acids into the loop connecting the swapped secondary structural elements. Characterization of IL- 10M1 by mass spectroscopy, size exclusion chromatography, cross-linking, and circular dichroism shows that IL-10M1 is a stable α-helical monomer at physiological pH whose three-dimensional structure closely resembles one domain of wtIL-10. As previously reported, incubation of wtIL-10 with a soluble form of the IL-10Rα (sIL-10Rα) generates a complex that consists of 2 wtIL-10 molecules and 4 sIL-10Rαs. In contrast, IL-10M1 forms a 1:1 complex with the sIL-10Rα. Characterization of the interaction using isothermal titration calorimetry confirmed the 1:1 stoichiometry and yielded a dissociation constant of 30 nM with an apparent binding enthalpy of -12.2 kcal/mol. Despite forming a 1:1 complex, IL-10M1 is biologically active in cellular proliferation assays. These results indicate that the 1:1 interaction between IL-10M1 and IL-10Rα is sufficient for recruiting the signal transducing receptor chain (IL. 10Rβ) into the signaling complex and eliciting IL-10 cellular responses.