Insulin-like growth factor I (KGF-I) gene encoding the prepro-form of IGF-I was isolated from human adult liver cDNA libraries. Mature form of IGF-I cDNA was fused with a truncated lacZ gene, using synthetic linker containing hydroxylamine cleavage site. In expressed cell lysates, galactosidase-IGF I fusion protein was about 30% of total cellular proteins. The fusion proteins which were purified with Sephacryl S200 column chromatography were treated with hydroxylamine. The released IGF-I was purified with SP-Sephadex C-25 column chromatography. The purified IGF-I was further purified using RP-HPLC.