The dilute prothrombin time (dPT) is a widely accepted sensitive screening test for the lupus anticoagulant (LA). In general, Simplastin (Organon Teknika), a rabbit brain thromboplastin, diluted 1/500, is used in this test. Recently, human tissue thromboplastin obtained by recombinant DNA technology has become available and we have evaluated the usefulness of one such preparation, Innovin (Dade), for the detection of the LA. dPTs, using several dilutions of Innovin were determined on plasmas from 18 normal individuals and 15 patients with a well-documented LA. The dPT ratios, calculated as the individual result divided by the mean normal, were statistically compared. Innovin in dilutions from 1/100 onwards was found to be significantly more responsive to the LA than Simplastin (P < 0.05). Intra-assay coefficients of variation (CVs) ranged from 1.05% to 14.8% with Innovin, versus 2.7%-24.3% with Simplastin (P < 0.05); inter-assay CVs ranged from 5.6%-11.8% with Innovin, versus 4.2%-33.8% with Simplastin (P < 0.001). Analysis of 316 consecutive plasma samples from non-anticoagulated patients, on which a LA determination was requested, showed a 100% sensitivity and a 96% specificity of the dPT performed with Innovin, as compared to 81% and 93% respectively for the dPT using Simplastin.