The most common inherited bleeding disorder in man, haemophilia A, is caused by defect in factor VIII, a component in the blood coagulation pathway. The X-chromosome-linked disease almost certainly stems from a heterogeneous collection of genetic lesions. Because, without proper treatment, haemophilia can be a fatal disease, new mutations are necessary to account for its constant frequency in the population. In addition, haemophilia A displays a wide range of severity, and some 15% of haemophiliacs generate high levels of antibodies against factor VIII ('inhibitor patients'). The present work elucidates the molecular genetic basis of haemophilia in some individuals. Using the recently cloned factor VIII gene as a probe, we have identified two different nonsense point mutations in the factor VIII gene of haemophiliacs, as well as two different partial deletions of the gene. Our survey of 92 haemophiliacs indicates no firm correlation between antibody (inhibitor) production and gross gene defects.