Secretory leukocyte protease inhibitor (SLPI) is a neutrophil elastase inhibitor which also has antibacterial and anti-inflammatory properties. It is found associated with mucosal membranes. Although SLPI has been reported in the cervix it has not thus far been reported in human endometrium. This study investigates the presence of SLPI in endometrium, first trimester decidua and trophoblast. Cultured first trimester decidua was found to produce 4. 7 +/- 2.0 ng/mg/24 h of SLPI. Endometrium and trophoblast were both found to secrete significantly lower amounts of SLPI (P < 0.01) although endometrial expression was menstrual cycle dependent with increased secretion in the secretory phase. Although relatively low concentrations of SLPI were released from the endometrium during culture, most of the SLPI remained associated with the tissue and could be recovered with mild acid extraction. This is in agreement with the high isoelectric point (pI) for SLPI, associated with high solubility at low pH. The main site of SLPI synthesis in endometrium and decidua was found to be the glandular epithelium. An antibiotic role for SLPI in the endometrium and decidua during implantation and pregnancy would be consistent with the expression profile and localization of SLPI.