Reactions of IgG antibodies (ordinate) from patients 1 (squares) and 2 (circles) with various quantities of PF4 (abscissa) bound to heparin immobilized by EL and XL to agarose beads. A ratio of 1 μg PF4/μL of EL beads was optimal for antibody binding. Antibody failed to bind XL heparin beads, even at a ratio of 0.625 μg PF4/μL of beads, about 2.5 times the saturating concentration. Values shown are the average of duplicate determinations. No detectable immunoglobulin binding occurred when either preparation was incubated with a total immunoglobulin fraction from normal plasma (not shown).