The patient was born in 1944; at the age of 7 he underwent a tonsillectomy, bled excessively and required a blood transfusion. He had dental extractions when he was 14 and bled for a week afterwards. At 25 he had a birthmark removed from his left shoulder and again bled excessively. At that time two-stage FVIII:C assays were performed and he was diagnosed as having mild haemophilia A with a FVIII:C level of 19%. 3 years later (in 1972) he presented with a large haematoma of the buttock after minor trauma and a two-stage assay reported a FVIII:C level of 11%. Interestingly, his APTT was always normal. In 1977 he had sebaceous cysts removed under the cover of FVIII concentrate. He received further concentrate in 1982 following an injury to his left thigh and 6 weeks later developed hepatitis (in retrospect hepatitis C). He has not had any further bleeding problems. At a recent routine visit his baseline FVIII:C level was checked with our (now routine) one-stage assay and found to be normal. Subsequently on three blood samples taken on different days we obtained the assay results shown in Table I. On the second of these samples the factor VIII antigen was measured by Immunozym ELISA (Techniclone, Dorking) and found to be 82 iu/dl.