Normal human sera (NHS), heat-inactivated at 56 degrees C for 30 min, demonstrated positive ELISA reactions for anti-cardiolipin (aCL) antibodies. The heat-induced reactivity in ELISA was inhibitable by the cardiolipin antigen and was abolished by prior IgG depletion of the heated NHS with a protein A preparation. The heat-potentiated aCL also cross-reacted selectively with phosphatidic acid and phosphatidylserine, but not with phosphatidylcholine or phosphatidylethanolamine.