Secretory leukoprotease inhibitor (SLPI), also called antileukoprotease (ALP), is a 12-kDa, nonglycosylated serine protease inhibitor present in mucous secretions. It is thought to play a role in protecting the mucosae from injury associated with inflammation. SLPI is locally produced by serous cells, including bronchial submucosal glands. Elafin and SLPI are members of larger families of proteins secreted predominantly at mucosal sites, and have been shown to be modulated in multiple pathological conditions. Elafin and SLPI are structurally related in that both have a fold with a four-disulfide core or whey acidic protein (WAP) domain responsible for inhibiting proteases. SLPI is a prominent innate immune protein of the respiratory tract, possessing serine protease inhibitor activity, antibacterial activity, and anti-inflammatory/immunomodulatory activity.