A vWf: Ag ELISA which uses biotinylated anti-vWf: Ag IgG and an avidin/biotinylated peroxidase (ABP) complex detection system has been developed. The sensitivity of the ELISA for vWf: Ag was 4 X 10(-5) U/ml which is approximately 10-fold lower than previously reported vWf: Ag ELISA's. The avidin used for the preparation of the ABP complex was glutaraldehyde-treated. This treatment increased the reactivity 2-3-fold as compared to ABP complex prepared using untreated avidin. The reactivity of two commercial detection reagents was also tested. The effect of the reduction of the total assay time to 3.5 h is also described. In this form the detection limit for vWf: Ag was 2 X 10(-4) U/ml, the intra-assay precision at vWf: Ag concentrations of 0.01 and 0.0025 U/ml was 3.8% and 4.9% respectively and ELISA results for normal individuals still correlated well with those obtained using electro-immunodiffusion (r = 0.93).