Mouse uterine draining lymph node (UDLN) cells, or soluble factors released by them in both phases of the estrous cycle were examined in the graft-vs-host (GVH) assay. The immune response during the estrous phase was significantly decreased as compared with diestrous phase; 24-hour culture supernatant of UDLN cells was also able to decrease GVH activity mounted in F1 hybrids. This difference in reactivity was not found when peripheral nodes (PN) or spleen cells were tested. These results show that in physiological conditions it is possible to detect an hormonal influence on T lymphocyte function and would suggest that a non-specific immunosuppression would appear even during the estrous phase.