Forty-three laboratories in 13 countries participated in a workshop to determine the degree of agreement between laboratories performing anticardiolipin tests. Each laboratory received freeze-dried aliquots of three samples labeled G1 (107 GPL units), G2 (20 GPL units), and G3 (6 GPL units) to be used as reference standards in the IgG assay, and three samples labeled M1 (106 MPL units), M2 (21 MPL units), and M3 (5 MPL units) as references for the IgM assay. Participating laboratories were divided into 8 groups and serum samples were exchanged between laboratories in each group. For IgG anticardiolipin, results were reported as: high, IgG positive for samples with optical absorbance readings exceeding G1; medium, IgG positive for samples with readings between G1 and G2; low, IgG positive between G2 and G3, and negative, if less than G3. In like manner, samples were defined as high-, medium-, or low-IgM positive, with reference to standards M1, M2, and M3. An index of agreement was computed to determine the degree of agreement between laboratories in each group. Interlaboratory agreement was excellent in each category assessed. For high positive and negative IgG and IgM results, the index of agreement exceeded 90%, and for medium and low positive results, agreement exceeded 75%. The overall index of agreement between laboratories exceeded 90%. The researchers conclude that the use of these six standards to obtain a semiquantitative measure of anticardiolipin positivity will enable good interlaboratory agreement in reporting anticardiolipin results.