A method of investigating the coagulation mechanism is described. Results obtained by this procedure are compared with those of tests carried out on venous blood obtained simultaneously from normal subjects and patients with a variety of clotting defects. Good correlation was found between the results of P and P tests on venous and capillary blood, and also between thromboplastin screening tests on capillary blood and antihaemophilic and Christmas factor levels. The methods described have proved reliable as a pre-operative screening procedure in routine use over a period of nearly two years.