A method of isolating and concentrating monospecific antibody from multispecific HL–A antiserum by adsorption with and elution from platelets is described. One unit of platelets containing approximately 1 × 1011platelets is sufficient to absorb one unit (300 ml) of serum of low titer. Antibodies directed against antigens determined at separate loci are readily separated by adsorption with appropriately selected platelets, whereas antibody specificities directed against antigens determined by the same locus are usually not separable. Successful adsorption and elution of an antibody present in duospecific antisera depends not only upon the presence of a particular HL–A antigen on the platelets used for adsorption but also upon the absence of any antigen group which cross‐reacts with the second antibody speci