Three families having more than one affected member with SLE or lupus-like disease were investigated by global coagulation tests as well as methods based on dilute thromboplastin, Russell's viper venom and thermal stability/absorption, and by RIA for anticardiolipin (CL) antibodies. Of the 19 persons, 11 had SLE or lupus-like disease. Eight of these 11 had a prolonged KPTT and other evidence of LA, while only 5/11 had high anticardiolipin titres. Four healthy spouses of affected females, and three asymptomatic siblings also had prolonged non-correctable KPTTs. These persons had no bleeding or thrombotic history and normal clotting factor levels. Further clotting tests were negative, although one had raised anti-CL antibody. Such cases may account for some of the patients one finds during routine haemostatic screening with unexplained prolonged KPTT. Although anticardiolipin levels are raised in subjects with LA, there was no close correlation between length of KPTT and anticardiolipin titre. These findings would support a hypothesis of transmissible agents or other environmental factors being involved in lupus-like disorders.