In 1994, the, French National Quality Control Group for Hematology, Etalonorme, conducted a large-scale interlaboratory survey concerning the detection of lupus anticoagulants (LA) involving all the 4,500 French laboratories. Each laboratory received the same batch of a lyophilized citrated plasma (94B3) prepared from a patient with LA that had been confirmed by all the techniques used in the intralaboratory study. In the interlaboratory survey, the screening test was activated partial thromboplastin time (APTT); mean APTT calculated from the results reported by 4,029 labs was prolonged (clotting ratio = 1.44) with a large dispersion (coefficients of variation = 18.8%). APTT of the mixture 94B3 + normal plasma were performed by 2,698 laboratories. No correction of APTT was obtained (R = 1.36, Rosner index = 24) with a wide variation between reagents (17 < Rosner index < 39). Only 15% of the participants performed confirmatory tests; dilute tissue thromboplastin inhibition test (TTI) performed by 509 laboratories gave 75% positive results. Tests with an increased amount of phospholipids (Staclot LA and Staclot PNP from Diagnostica Stago), used by 116 and 72 laboratories, gave 88% and 61% positive results, respectively. A total of 1,862 laboratories made the diagnosis of LA. The majority of those who failed in diagnosing LA used an APTT reagent largely used in France, containing kaolin. This survey allowed Etalonorme to inform French biologists and draft an educational program for the biologic detection of LA and the identification of its mechanism of action.