Minor Histocompatibility antigens (mHag) are responsible for the development of Graft- versus-Host-Disease (GVHD). They induce strong MHC restricted cytotoxic (CTL) and proliferative (Th) T cell responses which can be measured in vitro. In addition to CTLs, mHag Th cells play a role in the pathogenesis of acute GVHD. I will therefore aim at elucidating the nature of the MHC class II-restricted human mHag peptides and the proteins from which they originate. By immuno and biochemical isolation and purification procedures combined with sensitization of stimulator cells for T cell recognition, mHag peptide containing fractions responsible for stimulator cell sensitization will be identified. Subsequent mass spectrometric analysis will determine the amino acid sequence. Synthetic peptides generated accordingly should confirm the natural peptide sequence. Naturally, determination of the nature of MHC class II restricted mHag is clearly important in understanding the basis of the T cell responses these antigens evoke in transplantation.