Antigen presenting cells (APC) within murine decidual tissue in vivo have been shown to process the soluble antigen ovalbumin after intravenous administration and to present it in a form recognizable by immune T lymphocytes. In vivo antigen pulsed decidual APC stimulated T cell proliferation as efficiently as splenic APC and in an MHC restricted manner. In addition, anti-class II antibody plus complement treatment significantly reduced decidual antigen presenting capacity in vitro. These findings show that class II positive cells within the decidua can present antigen effectively in vivo and may therefore serve as APC for the presentation of fetal antigens to the maternal immune system during pregnancy.