Biotin-Des(1-6) IGF-II is prepared by a selective biotinylation procedure. As the non-biotinylated analog it has only very weak affinity to IGF-binding proteins. Binding to receptors is not substantially affected. Due to its strongly reduced binding affinity to the IGFBP?s, Biotin - Des (1-6) IGF-II may be used for detection of IGF-II receptors on western blots, in binding studies or in immunohistochemistry.