We have previously shown that oestradiol treatment of ovariectomized rats for 3 days inhibits antigen presentation by uterine stromal cells at a time when oestradiol increases the numbers of antigen-presenting cells (APC) in the uterine stroma. In the present study, we found that oestradiol treatment for 1 day is sufficient to inhibit antigen presentation by stromal cells. To define the mechanism(s) of this inhibition, we examined the effect of cytokines and found that exogenous transforming growth factor-β (TGF-β) inhibits antigen presentation when stromal cells from saline- but not oestradiol-treated animals are incubated with ovalbumin (OVA)-specific T cells and OVA. In contrast, antigen presentation by uterine epithelial cells was not affected by TGF-β. In other studies, the acute inhibitory effect of oestradiol (1 day) on stromal antigen presentation is fully reversed when anti-TGF-β antibody is added to the culture media. When given for 3 days, oestradiol inhibition of antigen presentation is partially reversed by anti-TGF-β antibody at a time when antibodies to tumour necrosis factor-α and interleukin-10 have no effect. To determine whether uterine epithelial cells produce TGF-β, epithelial cells were grown to confluence on transwell inserts. Our findings indicate that uterine epithelial cells produce biologically active TGF-β which is preferentially released basolaterally in the direction of underlying stromal cells. When oestradiol is given to ovariectomized rats 1 day before sacrifice, TGF-β production by epithelial cells increases within 24 hr in culture, relative to saline controls. Taken together, these results suggest that oestradiol inhibition of stromal cell antigen presentation is mediated through the stimulatory effect of oestradiol on TGF-β production by epithelial cells.