The expression of HLA-A and -B antigens on peripheral blood lymphocytes and blood platelets was measured using monoclonal antibodies in a semi-quantitative ELISA technique. Reactively of monoclonal anti-HLA-A2 and anti-HLA-B7, with lymphocytes as well as platelets, was in agreement with the presence of these antigens as detected by conventional HLA typing of lymphocytes. When the actual amount of HLA antigens was measured, a gene-dose effect was seen: cells from HLA-B7-homozygous individuals bound more monoclonal anti-HLA-B7 antibodies compared to their HLA-B7-heterozygous siblings. At the same time, cells of different donors showed only very small differences in binding of monoclonal antibody against an HLA-"backbone" determinant. Relative to total HLA-A, -B and -C expression, the amounts of HLA-A2 on lymphocytes and platelets were similar. On the other hand, the expression of HLA-B7 on platelets was diminished compared to that on peripheral blood lymphocytes.