The distinction between a specific factor inactivator and a non-specific inhibitor is important when confronted by a patient with a history of bleeding and abnormal in-vitro coagulation tests. We report on two patients who presented with bleeding and a prolonged activated partial thromboplastin time. Initial factor assays suggested combined deficiency of factors VIII and IX as a result of the presence of inactivators. The use of dilution studies, chromogenic assays, a novel in-house enzyme-linked-immunosorbent-assay-based technique and phospholipid neutralization, demonstrated that Case 1 had a genuine factor VIII inactivator resulting in factor VIII levels of less than 1 IU/dl but no factor IX deficiency. Case 2 had normal levels of factor VIII on further testing and no specific inactivator to either factor VIII or IX but a potent antiphospholipid antibody which had interfered with the phospholipid-dependent in-vitro assays. Care must be taken in the interpretation of laboratory assays in the presence of antiphospholipid antibodies to ensure that the correct diagnosis is made and inappropriate treatment avoided.