Cyclic peptides capable of activating the erythropoietin receptor (EPOR) were isolated from phage display libraries by screening with a novel EPOR-IgG fusion protein reagent. A parental clone ERB1 (EPO Receptor Binder 1) was first isolated from a phage display library displaying 38 random amino acids as an N-terminal fusion with the M13 minor capsid protein, pill. An evolved library was then produced from the parental sequence using an oligonucleotide saturation mutagenesis strategy which yielded EPOR binding sequences with 20 times the relative affinity of ERB1. Two synthetic peptides were constructed from these sequences both of which bind the EPO receptor in specific ELISA, and act as full agonists in EPO dependent cell proliferation assays. These peptides are 18 amino acids in length, disulfide-bonded, and have a minimum consensus sequence of CXXGWVGXCXXW, where X represents positions tolerant of several amino acids.