A method has been developed for the immunological quantitation of factor VIII:C Ag using a medium titre (50 new Oxford units) factor VIII:C antibody arising in a severe multitransfused haemophiliac. The method, which utilizes clotting inhibition in an agarose gel medium, gave close and significant correlation with the two-stage factor VIII:C procoagulant assay (r=0.83, P less than 0.01) for 54 normal subjects. Similar or higher values were found in 19 mild, moderate and severe haemophiliacs with 2-30% average normal plasma levels of factor VIII:C (r=0.72, P less than 0.01). Four mild von Willebrand patients gave similar results by immunoassay and procoagulant assay methods. A previously identified patient with cross reacting material (CRM+) gave an immunoassay within the normal range (66%) with only 4% VIII:C activity detectable. The method offers a simple, sensitive and apparently reliable procedure for the assay of plasma factor VIII:C Ag which may prove useful in the further investigation of factor VIII:C Ag and antibody heterogeneity. The procedure offers an alternative to immunoradiometric assay and may be of potential use in the assessment of the haemophilia carrier state and possibly the early detection of thrombosis.