This study was undertaken to investigate the influence of beta 2-glycoprotein I (GPI) on anticardiolipin antibody (aCL) titration in leprosy. The study group consisted of 140 sera from patients with multibacillary leprosy (46 borderline, 94 lepromatous). The group included newly diagnosed, previously untreated patients, patients under treatment and patients released from treatment. GPI addition enhanced significantly the aCL titres in sera from lepromatous leprosy but not in those from borderline leprosy. Moreover, when the patients were classified according to their bacteriological status, aCL titres were found to be significantly higher in skin smear positive patients compared to bacteriologically negative patients. Thus, the present study demonstrates that aCL in multibacillary leprosy patients are mainly of the GPI-dependent type and emphasizes the importance of GPI addition for aCL titration in leprosy.