Although an immunomodulatory role for estrogens has long been demonstrated by experimental and clinical observations, the mechanism by which estrogens exert their effect on T cells has not been clearly defined. In this study we analyzed the effects of β-estradiol (E2), at its contraceptive dose, on the delayed-type hypersensitivity (DTH) to purified protein derivatives (PPD) and associated immune response in female mice. E2 treatment decreased PPD-specific DTH response, which coincided with a decrease in the leukocytes numbers in the draining lymph nodes (DLN) and spleen compared with control mice. E2 treatment also suppressed the in vitro PPD-specific proliferative response of DLN and spleen cells from PPD-primed mice. The analysis of production and gene expression of cytokines by DLN cells demonstrated that E2 treatment suppressed IL-2 and IFN-γ production in response to PPD in vitro. In contrast, IL-4 and IL-10 gene expression by DLN cells of E2-treated mice, taken 24 h after in vivo restimulation of mice with PPD, was enhanced. Furthermore, we found that spleen APC from E2-treated mice failed to induce optimum proliferation of the PPD-primed T cells in response to PPD in vitro. The impaired APC function by E2 was not due to induction of suppressor cell activity because addition of the normal spleen APC to APC from E2-treated mice restored the proliferative response of the PPD-primed T cells in response to PPD. Our results suggest that the E2-mediated inhibition of DTH reaction is due to a combination of the down regulation of APC function and deviation of the immune response from Th1-type to Th2-type.