In order to assess thrombogenicity and liver toxicity of different coagulation factor concentrates, antithrombin III, soluble fibrin, alanine aminotransferase (ALAT) and gamma-glutamyl transpeptidase (GT) were measured in samples taken before, 30 min and 24 h after the infusion. Seventy-six studies were performed in 55 patients with haemophilia A (37), B (11) or von Willebrand’s disease, type III (7). A sharp rise of soluble fibrin was observed in 2 patients with haemophilia B, indicating that modern factor IX concentrates may still be thrombogenic. A slight increase was also seen after infusion of factor VIII-von Willebrand factor concentrates of low purity. Antithrombin III, ALAT and GT did not change significantly after any of the factor concentrates. The alterations in those parameters did not correlate with the impairment of liver function.