Dilution and absorption tests. (a) Immunoglobulin G binding of selected sera positive for both anticardiolipin antibody (aCL) and antimonolysocardiolipin antibody (aMCL) was enhanced by increasing the concentration of cardiolipin (CL) derivatives coated on the enzyme-linked immunosorbent assay (ELISA) plate wells. To further characterise the antigen specificity, three aCL/aMCL-positive sera were tested in ELISA after pre-absorption with increasing concentrations of CL derivatives. Pre-absorption with CL and monolysocardiolipin (MCL) significantly decreased the binding of aCL to CL (b) and of aMCL to MCL (c), respectively. No significant competitions were observed with dilysocardiolipin (DCL) and hydrocardiolipin (HCL) pre-absorption concerning the binding of both aCL and aMCL sera (b, c). Each data point represents the mean of triplicate determinations. OD, optical density.