Two murine monoclonal antibodies, raised against von Willebrand factor (vWF), were used to construct an enzyme-linked immunosorbent assay (ELISA), for quantitation of vWF antigen (vWFAg) in human plasma and platelets. This assay had a lower limit of sensitivity of 0.0001 IU/ml in buffer, and thus is one to two orders of magnitude more sensitive than other ELISA assays which have been reported. The intraassay, interassay and interdilution coefficients of variation were 4.1, 10.4 and 9.9%, respectively. In normal plasma (n = 20), the vWFAg level was 0.83 (range: 0.42-1.25) IU/ml. In normal washed platelets (n = 10), 0.35 (0.25-0.49) IU/10(9) platelets was found. In plasma obtained from various patient groups the following vWFAg levels (geometric mean and range) were observed: von Willebrand's disease (n = 19): 0.18 (0.02-0.77) IU/ml; patients with liver cirrhosis (n = 20): 3.73 (1.68-9.20) IU/ml; patients with pregnancy-induced hypertension (n = 20): 4.14 (2.28-7.44) IU/ml and patients with malignant disease (n = 10), 2.54 (1.51-5.60) IU/ml. A linear correlation was found between vWFAg levels measured with a polyclonal antibody based Laurell electroimmunoassay (r = 0.92, n = 58) or with a polyclonal antibody based ELISA (r = 0.94, n = 64). The present assay is based on stable and reproducible reagents and allows the specific measurement of vWFAg in plasma and in platelets. This assay may constitute a useful tool for the further investigation of clinical conditions associated with changes in vWFAg levels. In addition, its high sensitivity may facilitate a more detailed study of platelet vWFAg in normal and in pathological conditions.