An enzyme linked immunosorbent assay (ELISA) has been developed to measure VIII:Ag in plasma and concentrates. The assay utilizes two commercially available monoclonal antibodies to VIII:Ag and provides an alternative to the established immunoradiometric assay (IRMA). It has the advantage of not requiring the use of radioactive material and human antibodies. The assay sensitivity is 0.006 u/ml and the interassay coefficient of variation is 6.3%. Forty-eight samples with VIII:Ag levels ranging from 0.006 to 1.5 u/ml were assayed by both ELISA and IRMA. The coefficient of correlation between the two assays was 0.89. In addition to measuring human VIII:Ag, it is also possible to detect antigen in several animal plasma and sera.