Various procedures for the solubilization of the imipramine-binding protein (IBP) of human platelets were compared. An IBP of a molecular weight of 300 000-400 000, as determined by exclusion chromatography on Sepharose 6B, was obtained with high amounts of digitonin and with lysolecithin. With smaller amounts of digitonin the molecular weight varied between more than 1 million and about 550 000 depending on the batch of detergent. The KD values and the IC50 values of drugs inhibiting imipramine binding were similar in the soluble preparation and in intact membranes. CHAPS and CHAPSO, even in high amounts, yielded solubilized IBP of high molecular weight (greater than 1 million). Membrane preparations of human platelets solubilized with high amounts of digitonin and with lysolecithin would therefore seem to be the most suitable for further purification of IBP.