A selective screening program has been established to identify rapidly and effectively the fine specificity of HLA-DR antibodies in pregnancy anti-HLA sera. Following initial HLA-A, B, C screening sera with extra- or multispecific reactions were selected and specifically tested after platelet absorption on isolated B- and T-lymphocyte populations of the serum donor's husband. Identification of the HLA-DR serum. Multispecific anti-DR sera were defined and rendered operationally monospecific by titration. Some critical steps in a reliable assessment of HLA-DR typing reagents could be worked out. Weak HLA-A, B antibodies, B-cell auto- and Lewis antibodies may cause positive reactivity preferentially or even selectively on B lymphocytes. Of particular importance was the hidden presence of HLA-C specific antibodies, since they cannot be absorbed out of stored platelets. In addition they are not readily detectable through screening on typed panel cells. Because of the frequently very high linkage disequilibrium between HLA-B and HLA-C alleles it is difficult to select appropriately dissecting panel cells. The two points demonstrated above gain even more weight when isolated T and B cell populations are used for HLA-DR typing, because HLA-C antibodies preferentially kill B cells. In this fashion contaminating HLA-C antibodies are not only difficult to detect but can mimic the presence of HLA-DR antibodies.