Lupus anticoagulants (LA) are associated with an increased risk of thrombosis and laboratory detection is of major importance. Various tests are available for LA screening and confirmation, but they differ in sensitivity and specificity, frequently lacking the ability to discriminate between the presence of LA, heparin and oral anticoagulants. We noticed that a patient with LA who had a prolonged activated partial thromboplastin time (APTT) by our routine method, gave a normal result with a different APTT reagent. This latter reagent, which contained soy bean phosphatides (SBP), was compared with a reagent containing rabbit brain phospholipids complexed with kaolin (RBK), for APTT measurement in a variety of patients. There was no significant difference in APTT ratio between the two reagents in plasma samples from healthy normal subjects. In LA samples, SBP gave consistently lower APTT ratios than RBK (mean +/- SEM, 1.04 +/- 0.05 and 2.08 +/- 0.19 for SBP and RBK respectively; P < 0.001). In LA patients receiving oral anticoagulants for antithrombotic prophylaxis or treatment, the APTT ratio was again significantly shorter with SBP (1.60 +/- 0.17 and 3.40 +/- 0.67; P < 0.05). In LA negative patients receiving oral anticoagulants, the relationship was reversed, and a higher APTT ratio was obtained with SBP than RBK (1.61 +/- 0.13 and 1.31 +/- 0.12; P < 0.001). In addition, there were no significant differences in APTT ratios for the two reagents when samples from patients receiving heparin therapy, or patients with acquired factor VIII deficiency or inherited deficiency of factor VIII or IX were studied. The use of the SBP reagent alongside a LA sensitive APTT reagent allows a rapid screening for LA, as well as a confirmation of the phospholipid dependency of the inhibitor.