Antithrombin 111 (ATIII) is the major physiological inhibitor of thrombin. ATIII functions by providing an exposed reactive site which binds thrombin stably, a process greatly enhanced by heparin (Lane et al., 1992). ATIII deficiency (McK. No. 1073001, which occurs at a frequency of between 1/5,000 and 1/2,000 in the general population (Beresford, 1988), is present in-3% of patients presenting with recurrent venous thrombosis (Gladson et al., 1988). Two main types of ATIII deficiency have been distinguished (Lane et al., 1993): In type I deficiency, both ATIII activity and antigen are reduced to the same extent whereas in type I1 deficiency, a higher level of ATIII antigen than activity is evident. We have used Single Strand Conformation Polymorphism (SSCP) analysis to screen for mutations in the seven exons of the antithrombin 111 (AT3) genes (Bock et al., 1988) of patients with ATIII deficiency.