The objective of this study was to determine whether thymus cells present antigen and if endocrine balance influences antigen presentation. We report here that antigen presenting cells (APC) from the thymus glands of male and female rats, when incubated with ovalbumin (OVA)-specific T cells and OVA, are functionally able to present antigen via MHC class II. To determine whether antigen presentation in the thymus is under hormonal control, tissues from female rats at different stages of the estrous cycle were analyzed. Antigen presentation was higher at estrus and proestrus than that seen at diestrus when estradiol levels are low. Estradiol given to ovariectomized animals for 3 days stimulated antigen presentation by adherent thymus cells compared to saline controls. Flow cytometry studies indicated that the adherent thymus cell preparations consisted of DC, T cells, B cells and cells of the myeloid lineage all of which expressed MHC class II, as did a small population of non-leukocytes. Antibody neutralization studies indicated that thymus cell antigen presentation involves the expression of transmembrane proteins B7.1 and B7.2. These studies demonstrate that sex hormones play a central role in regulating antigen presentation in the thymus.