The solubility of granulopoiesis inhibiting factor (GIF) in various aqueous and organic solvents was investigated. GIF is soluble in water, 10% acetic acid, and 10% pyridine. It is not extractable by 1-butanol at low and high pH. A high solubility was found in polar organic solvents (ethanol-acetone 9:1, and chloroform-methanol 1:1), whereas GIF seems to be insoluble in pure chloroform.