Increased incidence of infections, cancer, monoclonal gammopathies and rheumatic diseases in aged humans has been described. Histocompatibility antigens are involved in the regulation of immune response and it has been suggested that age-related alterations in the murine immune system may be due to changes in the expression of these antigens on the immunocompetent cells. In this paper we study the expression of HLA-DR/DP and HLA-DQ antigens on monocytes from healthy human elderly donors. Results show that peripheral blood monocytes from elderly subjects express decreased levels of HLA-DR/DP antigens (61.5 + 16.3) when compared to young controls (82.5 + 8.5) and increased levels of HLA-DQ antigens (40.4 + 16.8 and 23.6 + 7.1, respectively). The abnormal levels of expression of HLA-class II molecules could be related to the altered immune functions observed in elderly people.