A precise and sensitive chromogenic assay for the measurement of very low levels of factor VIII (FVIII) in plasma has been developed. The assay is based on modifications of a commercially available chromogenic assay. The modifications include reduction of sample final dilution factor and prolongation of the development period. The modified assay allows accurate and precise measurement of FVIII in the range of 0.001–0.02 IU mL−1. The detection limit is 0.0005 IU mL−1 and the quantitation limit is 0.0015 IU mL−1. This assay can be used in research and to study the clinical efficacy of low circulating levels of FVIII in haemophilia A patients.