The primary anti-phospholipid syndrome is characterized by recurrent venous and arterial thromboembolic phenomena, recurrent fetal loss, thrombocytopenia, and serological evidence of anti-cardiolipin (aCL) antibodies or/and the presence of lupus anticoagulant (prolonged activated partial thromboplastin time). The exact role of aCL antibodies in pathogenesis is not clear and the mechanism by which the antibodies may induce the various manifestations is unknown. In the current study we evaluated the effect of passive transfer of aCL antibodies (to the tail vein of naive mice) on fecundity, fetal loss (fetal resorption), and the weight of embryos and placentae. Two types of aCL antibodies were employed: (i) mouse monoclonal aCL antibodies derived from a BALB/c mouse in which experimental systemic lupus erythematosus was induced by a pathogenic idiotype (idiotype 16/6) of anti-DNA antibodies and (ii) polyclonal IgG and IgM aCL antibodies derived from serum of a patient with primary anti-phospholipid syndrome. After infusion of either antibody (10 micrograms per mouse) we could demonstrate lower fecundity rate, increased resorption index of embryos (equivalent to recurrent fetal loss), lower number of embryos per pregnancy, and lower mean weights of embryos and placentae in comparison to mice infused with appropriate control immunoglobulins. We conclude that the aCL antibodies may have direct effects on fecundity and on the outcome of pregnancy.