Leptin plays a role in regulating the body weight in mice. Injection of recombinant mouse leptin expressed in Escherichia coli reduced the food intake and body weight in normal, ob/ob and diet-induced obesity mice. Hyperglycemia, hyperinsulinemia and hypothermia can also be corrected in ob/ob mice after leptin injection. Leptin is a 16-kDa secretory protein comprising 167 amino acids produced in adipose tissue and is secreted to blood stream. In this study, a recombinant mouse leptin was generated and purified from a baculovirus expression system. This protein was used to identify putative ligands using a phage library of random peptides. Three leptin-binding phage clones were found, which were characterized by DNA sequencing and ELISA methods. The amino acid sequences of the reactive peptides are: LAYCSDPVRCLVWWY, MFWISAVSFVDHALV and LVLVLSAFLCCGVG. All three clones bound to recombinant human and mouse leptins. These peptides may be useful tools to study leptin-receptor interaction, food intake and body weight regulation.