A sandwich enzyme immunoassay for plasma factor VIII related antigen has been developed which exploits a para-iodophenol enhanced chemiluminescent reaction to detect the horseradish peroxidase label. The assay entailed 15 min incubations with sample and with conjugate and had a detection limit of 0.12 mU. It showed good within batch precision (coefficient of variation = 2.95-5.8%) and results on a series of 57 specimens agreed with results obtained by immunoelectrophoresis (correlation coefficient = 0.97).