We report the functional phage display of single chain human interleukin-5 (scIL-5) and its use for receptor-binding epitope randomization. Enzyme-linked immunosorbent assays and optical biosensor analyses verified expression of scIL-5 on the phage surface and binding of scIL-5 phage to interleukin-5 receptor alpha chain. Furthermore, an asymmetrically disabled but functional scIL-5 mutant, (wt/A5)scIL-5, was displayed on phage. (wt/A5)scIL-5 was constructed from an N-terminal half containing the original five charged residues (88EERRR92) in the CD loop, including the Glu89 and Arg91 believed key in the alpha chain recognition site, combined with a C-terminal half containing a disabled CD loop sequence (88AAAAA92) missing the key recognition residues. This asymmetric variant was used as a starting point to generate an scIL-5 library in which the intact 88-92 N-terminal CD loop was randomized. From this epitope library, a receptor-binding variant of IL-5 was detected, (SLRGG/A5)scIL-5, in which the only charged residue in the CD loop is an Arg at position 90. Characterization of this variant expressed as a soluble protein in E. coli shows that the IL-5 pharmacophore for receptor alpha chain binding can function with a single positive charge in the CD loop. Charge-depleted CD loop mimetics of IL-5 suggest the importance of charge distribution in functional IL-5 receptor recruitment.