Small molecule-immobilized affinity matrices are indispensable tools in chemical genomics to screen and purify protein targets for biologically active small molecules. Usually, prior to immobilization, small ?molecules have to be derivatized at a position that does not significantly abrogate the intrinsic biological activity, or chemically synthesized to have an appropriate functional group for the immobilization chemistry. Here, we describe a photo-cross-linking technique to immobilize biologically active small molecules for protein target screening, without the need for chemical derivatization.