ABSTRACT: In the current series of experimental studies we show that anticardiolipin antibodies (ACA) are pathogenic: Infusion of serum ACA to the tail vein of naive mice induces experimental antiphospholipid syndrome (APLS) characterized by thrombocytopenia, prolonged aPTT, and recurrent fetal resorptions. Similar experimental APLS is induced by active immunization with serum as well as with natural human monoclonal ACA. APLS is also associated with low fecundity rate. The experimental APLS models were employed to demonstrate the efficacy of aspirin, low molecular heparin, and interleukin-3 preventing recurrent fetal loss. In another experiment, immunization with human monoclonal anti-DNA antibody was followed by the induction of APLS secondary to experimental systemic lupus erythematosus (SLE). In all studies, IgGs were found to be more pathogenic than IgMs ACA. These studies confirm the autoimmune nature of APLS.