The presence of HLA (class I) determinants on red blood cells was found in 15% of individuals of the French population. This percentage varies considerably from one geographical region to another in the world. The red blood cells (RBC) that express class I determinants also express beta 2 microglobulin but not HLA-DR antigens. This phenomenon does not appear to be linked to the erythrocyte phenotype nor to the HLA phenotype of the subject. Cytofluorometry analysis allowed us to establish that the large individual variations in the expression of the HLA antigens on RBC are related mainly to the percentage of positive cells but not to the fluorescence intensity of each cell. A comparative study between platelets and red blood cells showed that the expression (i.e. antigen density) of HLA class I determinants is weaker on red cell membrane.