A ntiphospholipid syndrome (APLS) is characterized by thrombocytopenia, thromboembolic phenomena, and recurrent fetal loss, associated with anticardiolipin antibodies (ACA) and/or lupus anticoagulant. The syndrome may be primary or may be associated with other conditions such as systemic lupus erythematosus. We have previously shown the ability to induce APLS in naive mice following passive transfer of serum and monoclonal ACAs. Similarly we generated the secondary APLS in BALB/c mice following immunization with a pathogenic anti-DNA antibody. In the current study we report on the induction of primary APLS following immunization of BALB/c mice with a human monoclonal ACA (H-3). The mice developed high persistent titers of ACA. The APLS was characterized by prolonged activated partial thromboplastin time, low fecundity rate (21% vs. 48% of control immunized mice), high resorption index of fetuses (25% vs. 3%), and low weights of embryos and placentae. Our study points to the ability of inducing primary APLS in naive mice. The induction of various presentations of APLS by different ACA may explain the diversity of clinical manifestations seen in patients with APLS.