A human alloantiserum produced by multiple immunizations of subject BC with lymphocytes from donor MF was analyzed by quantitative absorption and elution techniques. The antiserum demonstrated three distinct patterns of reactivity with lymphocytes from a donor cell panel. Of these three classes of reactivity, only one class showed absolute correlation with an identifiable HL-A specificity, HL-A3. By absorption and elution criteria the serum appeared “monospecific”; complete absorption was attained by cells not carrying HL-A3 and eluates from these cells had the same specificity as the unabsorbed serum. Three additional HL-A antisera were used for comparison and gave similar results.