Human secretory leucocyte proteinase inhibitor (SLPI) is a low-molecular weight, acid-stable inhibitor of polymorphnuclear granulocyte elastase and cathepsin G. Previous reports have demonstrated the existence of SLPI in the respiratory tract, salivary glands and cervical mucosa. Positive staining for SLPI using immunohistochemical techniques has been reported in serous glands in nasal mucosa. We now confirm this observation and show, using in situ hybridization, that the pattern of expression of mRNA corresponds to the distribution of the encoded protein, SLPI. This, together with the high concentration of SLPI in nasal secretions, confirms the hypothesis of a local production of SLPI in the mucous membranes.