Focuses on natural antimicrobial peptides (AMPs) with defined sequence and activity. APD includes a total of 2619 AMPs with 261 bacteriocins from bacteria, 4 AMPs from archaea, 7 from protists, 13 from fungi, 321 from plants and 1972 animal host defense peptides. The APD3 contains 2169 antibacterial, 172 antiviral, 105 anti-HIV, 959 antifungal, 80 antiparasitic and 185 anticancer peptides. Newly annotated are AMPs with antibiofilm, antimalarial, anti-protist, insecticidal, spermicidal, chemotactic, wound healing, antioxidant and protease inhibiting properties. A unique and powerful feature of the APD3 is that its search interface consists of a pipeline of search functions. The more one selects or enters, the less one will get. Without activating anything in the interface, a simple search returns all the peptides in the database. The users can filter the information freely at their will. To make it more convenient, the APD3 has enhanced the search interface by re-grouping the existing search icons into functional zones based on information types and by adding new search icons for peptide activity.