To determine the expression of macrophage inhibitory protein-3beta (MIP-3beta), a potential chemoattractant for endometrial natural killer (NK) cells, in the human endometrium.An experimental study.University department of obstetrics and gynecology.Thirty-seven fertile women with regular menstrual cycles and nonpathological endometrium, undergoing hysterectomy.Endometrium was obtained from operative samples.Paraffin-embedded endometrium was immunostained to determine the localization of MIP-3beta. The number of NK cells was counted in 10 nonoverlapping stromal areas. The MIP-3beta concentration in the homogenized endometrium was determined by ELISA.Immunoreactivity for MIP-3beta was observed in the surface epithelia, glandular epithelia, and stroma with some menstrual cycle-dependent fluctuation. The MIP-3beta concentration was significantly higher in the late secretory phase than in the other phases. It showed a trend toward correlation with the number of endometrial NK cells.MIP-3beta was expressed in the human endometrium, but our results could not strongly support the hypothesis that MIP-3beta is a potential chemoattractant for endometrial NK cells.