In this study we investigated the frequency and the type of manifestations of antiprothrombin antibodies (aFII) in patients with antiphospholipid syndrome (APS). In 16 (84.2%) of 19 patients with lupus anticoagulant (LA) and either anticardiolipin antibodies or antiphosphatidylserine antibodies, two types of abnormal patterns were shown on a crossed immuno-electrophoresis technique using anti-human prothrombin murine IgG. The slow-moving peak of prothrombin was detected in 8 patients, while a peak in the other patients had the slow-moving shoulder. These slow-moving materials might represent prothrombin-aFII complexes. In 13 patients who were studied on Western blots, IgGs of 11 patients (84. 6%) bound to human purified prothrombin, and the IgGs of 7 (53.8%) of these patients also bound to human purified alpha-thrombin. Our results indicate that aFII detected in patients with APS may explain part of the mechanism of LA.