A2M is a 718,000 KDa broad spectrum plasma protease inhibitor whose production by the human endometrium was recently reported. The multifunctional A2M receptor, also known as low density lipoprotein receptor-related protein, LRP, was also recently immunolocalized to the endometrial stroma. The objective of this study was to further characterize the site of expression of A2M and its receptor in order to gain more insight into their functional significance in the endometrium. Formalin fixed paraffin embedded human endometrium from hysterectomy and endometrial biopsy specimen was used for in situ hybridization analysis with S35 labelled riboprobes representing subcloned LRP and A2M cDNA fragments. Duplicate sections of human endometrium were hybridized with sense and antisense probe and coated with photographic emulsion. Resultant autoradiograms were analyzed qualitatively by light and darkfield microscopy, and quantitatively by a computerized analysis of the signal intensity. Immunohistochemistry for A2M was performed on some sections using an affinity purified polyclonal antibody. Expression signals for A2M were more numerous and intense in the secretory endometrium compared to proliferative endometrium. Endothelial cells lining the endometrial blood vessels appeared to be the main source of A2M expression. The A2M expression signals in secretory endothelium were 2-3 fold stronger than the proliferative endothelium suggesting transcriptional activation of A2M expression in the secretory endothelia. Rare glandular expression was observed in few secretory samples and confirmed by immunochemistry. Diffuse and uniform LRP expression was observed in the stroma of secretory and proliferative endometrium. The endometrial glands did not express LRP. Our findings suggest that A2M and LRP expression by the endothelium and stromal cells respectively may be involved in endometrial physiology and in controlling trophoblast invasion during the early phases of implantation.