Defensin HNP-3 is a peptide secreted by human polymorphonuclear leukocytes (PMNs) that has antimicrobial properties. It induces lysis of mammalian cells when used at a concentration of 25 μg/mL. It also inhibits growth of E. faecalis (ED50 = 100 nM) and clinical isolates of P. aeruginosa (MIC90 = 4 μM). HNP-3 binds to recombinant HIV-1 envelope glycoprotein (gp120) and human CD4 (Kds = 52.8 and 34.9 nM, respectively). It also binds to recombinant, immobilized human surfactant protein D (SP-D; Kd = 55.7 nM) and inhibits focus formation in Madin-Darby canine kidney (MDCK) cells infected with influenza A virus (IAV).