In order to investigate whether the endometrium of women with unexplained infertility differs immunologically from the endometrium of normal fertile women, a panel of six monoclonal antibodies was used to characterize the presence of the beta 1-integrins or very-late-activation antigens (VLA) in the different endometrial compartments. Precisely timed endometrial biopsies at 4, 7, 10 and 13 days following the luteinizing hormone surge were obtained from 24 normal fertile women (group I) and 24 women suffering from unexplained infertility (group II). Frozen sections were labelled using an avidin-biotin peroxidase technique. VLA-1, VLA-2 and VLA-3 were present in glandular epithelium, stromal cells and vessels of both groups. VLA-4 was detected in group I but was absent from glandular and surface epithelium of group II. VLA-5 was not present in any of the specimens. VLA-6 was identified primarily in the basement membrane of vessels, glandular and surface epithelium in both patient groups. This study indicates that most beta 1-integrins are present in endometrium throughout the luteal phase of the menstrual cycle. The differences observed between the two groups may contribute to unexplained infertility.