Abstract: Identification of the molecular targets of a drug or toxin is the first step in understanding how the drug or toxin works, an important advance in learning how to improve a drug or assess the risks due to a toxin. The primary action of a drug usually involves binding to a protein; secondary actions may express themselves in the form of side effects and in some cases may be due to binding to other proteins. Consequently, it is useful to identify all physiologically relevant sites of action of a drug or toxin. A simple method for obtaining a list of the potential targets of a drug, toxin or other biologically active substance (referred to collectively as ligands) involves a multistep process. The first step is screening a protein or peptide library to identify library members that exhibit high affinity for a particular ligand. The second step involves searching of sequence data bases for proteins that contain the sequences of the library members shown to have high affinity for the ligand.