This study reports the discovery and steroid hormonal regulation of a novel glycosylated form of NKp30 in human endometrial epithelium. NKp30 is a member of the immunoglobulin superfamily and one of three existing natural cytotoxicity-triggering receptors. NKp30 is a glycosylated protein and is thought to be selectively expressed in resting and activated natural killer cells. The aims of the present study were to fully characterize NKp30 mRNA and protein in human endometrium during the menstrual cycle, and to investigate the hormonal regulation of NKp30. NKp30 mRNA was significantly up-regulated in fresh tissues during late secretory phase of the menstrual cycle. Interestingly, NKp30 mRNA was also present in clonally derived endometrial epithelial cells (CEE) in comparable amounts to fresh tissue. NKp30 protein was predominantly found in the endometrial glands and luminal epithelia of the secretory phase endometrium. Western blotting and de-glycosylation studies show that a novel glycosylated form of NKp30 is present in endometrial epithelium and that it can dimerize. Further phenotyping of CEE by flow cytometry revealed that they are CK8(+)CD49f(+)NKp30(+)CD45(-)CD56(-). The data also show that transcription and translation of the novel form of NKp30 can be induced by progesterone treatment after 48 h in endometrial explants in vitro. This is the first report to show the presence of both NKp30 mRNA and a novel glycosylated form of NKp30 protein in endometrial epithelial cells.