We conducted this study to confirm whether or not Ca2+-dependent antiprothrombin antibody (aPT) in patients' sera with systemic lupus erythematosus and/or primary antiphospholipid syndrome would react to prothrombin/phosphatidylserine in the presence of Sr2+, Mn (2+) or Ba2+, or divalent metal ions like Ca2+, utilizing ELISA, and to analyze the clinical significance of these metal-dependent aPT. We found the presence of Bat(2+)- and Sr2+-dependent IgG, IgM and IgA-aPT in up to 65% of patients negative for Ca2+-dependent IgG-, IgM and IgA-aPT. Maximally 69% of them were complicated by antiphospholipid syndrome-related symptoms. These data suggest that the measurement of Ba2+- and Sr2+- along with Ca (2+)-dependent aPT may become a clinically useful tool to correctly detect patients with antiphospholipid-related complications. However, further study is needed to clarify the clinicopathological differences among these aPTs in the future.