SummaryThe author reviews what kind of information is brought over by the method of direct compatibility of bloods in the practice of blood transfusions. The test must be tried at different temperatures, in saline solutions as well as in serum. Finally a Coombs-test will complete the essay. Of course this method is time consuming but the author feels that it makes possible to avoid any kind of danger resulting from acquired incompatibility produced either by previous blood transfusions or by hetero specific fœtal unmunisation.