Summary. The adsorption on tricalcium phosphate and on barium sulphate of factor VIII from bovine and human plasma and from human factor VIII concentrates was studied. A technique for the preparation of a concentrate of bovine factor VIII by direct adsorption of plasma with tricalcium phosphate and then filtration on agarose gel has been developed. The use of EACA, glucose and glycine markedly increase the yield and make possible the lyophilization of highly purified factor VIII. The technique can be used on a preparative scale using 10–30 litres of bovine plasma.