Human serum albumin (HSA) has two primary binding sites for drug molecules. These sites selectively bind different dansylated amino acid compounds, which due to their intrinsic fluorescence have long been used as specific markers for the drug pockets on HSA. We present here the co-crystal structures of HSA in complex with six dansylated amino acids that are specific for either drug site 1 (dansyl-L-asparagine, dansyl-L-arginine, dansyl-L-glutamate) or drug site 2 (dansyl-L-norvaline, dansyl-L-phenylalanine, dansyl-L-sarcosine). Our results explain the structural basis of the site-specificity of different dansylated amino acids. They also show that fatty acid binding has only a modest effect on binding of dansylated amino acids to drug site 1 and identify the location of secondary binding sites. (C) 2010 Elsevier Inc. All rights reserved.