This study was undertaken to compare various extraction methods for insulin-like growth factor-binding proteins (IGFBPs) from insulin-like growth factor-I (IGF-I) in rat serum systematically, before measurement of IGF-I by radioimmunoassay (RIA). The values obtained in the IGF-I RIA following acid–ethanol (AE), acid-ethanol cryoprecipitation (AEC) and formic acid-acetone (FA) extraction methods were compared with the IGF-I values obtained following high-performance liquid chromatography (HPLC), which was the reference method. Radio-ligand blots were used to determine the pattern and degree of IGFBP removal by these methods. Over a wide range of circulating IGF-I levels, AE and AEC extraction gave IGF-I levels comparable with those obtained following HPLC. FA extraction resulted in IGF-I levels that were consistently higher (P P Journal of Endocrinology (1992) 134, 169–176