[Radioimmunoassay of antihemophilic factor A].

A radioimmunological method for the estimation of anti-haemophilic factor A was perfected and studied, making it possible to measure factor VII antigen as distinct from procoagulant factor VIII activity. The method uses a rabbit anti-factor VIII antibody, purified and labelled with I125. The technique of estimation is based upon the differential precipitability of specific factor VIII antigen-antibody complexes, which precipitate with 25 per cent saturated ammonium sulphate and of free antibody which does not precipitate under these conditions. The method is reproducible and sensitive, with the possibility of measuring 0.3 per cent of factor VIII antigen, i.e. 0.003 units/ml. Its specificity is confirmed by the absence of factor VIII antigen in a patient suffering from a severe form of von Willebrand's disease. Using this technique, normal subjects were found to have 96 +/- 31 per cent of factor VIII antigen in comparison with a pool of 25 normal plasmas. Haemophiliacs tested had, as would have been expected, antigen levels close to normal.