Using immunohistochemistry with the newly available monoclonal antibody MEM-G/1 the reaction patterns on frozen and formaldehyde-fixed paraffin-embedded sections on human placentas, lymph nodes, skeletal muscles, and kidney and liver allografts were compared. HLA-G (a nonclassical major histocompatibility complex class I molecule that is assumed to influence the immune response during pregnancy and some pathological conditions) was found within human extravillous cytotrophoblast but not within villous cytotrophoblast and placental mesenchymal tissue. No HLA-G expression on human lymph nodes, tonsils, and kidney and liver allografts was demonstrated. However, HLA-G expression was observed in all samples of skeletal muscle. The binding capacity of monoclonal antibody MEM-G/1 provides new possibilities to study physiological and pathophysiological roles of HLA-G in humans.