Heat treatment of normal human serum reveals otherwise masked anti-cardiolipin antibodies (aCL). We studied the mechanism of masking and the nature of the inhibitor of these aCL IgG. Other forms of treatment, besides heating for 30 min at 56 degrees C, can also unmask hidden aCL IgG. These include acid pH, hypermolar buffers and phospholipase digestion. When unmasked, these aCL recognize other anionic and zwitterionic phospholipids, but do not react with DNA, cell antigens or IgG. Using thin layer chromatography we demonstrate that the heat-labile inhibitor(s) of these aCL are phosphatidylserine, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylcholine. These antibodies are not beta2-glycoprotein-I dependent and actually compete with this protein for phospholipid binding. The hidden antibodies are comprised of two populations of IgG autoantibodies: one reactive with cardiolipin, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, phosphatidylethanolamine and sphingomyelin, and the other reactive almost exclusively with phosphatidylcholine and phosphorylcholine on enzyme-linked immunosorbent assay plates or when exposed by bromelain on the erythrocyte surface. Our data suggest that hidden aCL are natural oligoreactive IgG anti-phospholipid autoantibodies that circulate masked by their antigen.