An immuno-chromatographic method of eluting HL-A antibodies from platelets has been described. An anti-HL-A3 serum (BC) was absorbed with platelets immobilized on a nylon column. The platelets obtained from HL-A3 (class I), HL-All (class II) or HL-Al (class III) positive donors were used to absorb HL-A antisera. A linear pH gradient (pH 7.4 to 2.5) was used to elute the antibody from the platelet columns. Anti-HL-A3 activity was eluted from class III platelets at pH 5.0, from class II platelets at pH 4.5 to 3.7, and from class I platelets at pH 4.2 to 2.7. It was shown further that the eluates from class I and class II platelets had different qualitative and quantitative serologic activities when they were retested on lymphocytes obtained from each class of donor. These data are indicative of differences in antibody avidity for each class of platelets analyzed and suggest that these differences are due to cross-reactivities. It has also been shown that the avidity of anti-HL-A3 antibody from early and hyperimmune bleedings increases with repeated immunization. These experiments illustrate the heterogeneity of HL-A antibodies.