HLA class I molecules were quantitated on erythrocytes from individuals expressing either high or low levels of such antigens. Quantitative determinations were accomplished using 125I-labeled Fab fragments of the anti-HLA monoclonal antibody W6/32 in a competitive binding assay. The experimental conditions of the test system were established using red cells from an individual found to express high levels of red cell HLA when examined by flow cytometry. The competitive binding assay met the requirements of ligand specificity and specific binding saturability. Scatchard analysis revealed that there were 1684 +/- 39 (mean +/- SD) HLA molecules/red cell. In two other donors in whom erythrocyte HLA was undetectable by flow cytometry specific binding of the 125I-W6/32 Fab fragments was clearly demonstrated, indicating the presence of HLA on red cells of these donors as well. The number of HLA molecules/red cell was estimated to be between 100 and 200 for these individuals. Thus, in a blood transfusion unit, the number of HLA molecules contributed by the red cells is comparable to that of the leukocytes. Blood highly depleted of leukocytes and platelets and selected from donors with low amounts of red cell HLA was not beneficial (when transfused to selected patients) in that their sensitizing effects were not significantly different from regular blood transfusions. These results show that the amount of HLA antigens on red cells, while low if compared with other cell types, is significant in terms of the absolute antigenic content of blood transfusions. They also show that transfusion of blood units containing HLA antigens in concentrations as low as can be achieved with current technology were not useful in preventing HLA sensitization in patients at risk.