BLOOD displaying positive serological reaction for syphilis and erythrocytes of blood stored for more than 21 days are usually discarded in blood banks. The erythrocytes of such blood samples were utilized for the preparation of erythrocyte powder. They were washed with saline and haemolysed with distilled water. The stromata were then washed three times with distilled water cooled to 6° C. Thereafter, the stromata sediment was lyophilized and pulverized in a mortar. The drying could also be carried out at 37° C., the stromata being spread in a thin layer ; but the lyophilized preparation was more active. About 1,500 mgm. of fine, pink powder was obtained from 200 ml. of packed red cells. The powder could be stored at 4° C. for at least two months without any appreciable loss of activity. Before use, the requisite amount of the powder was weighed out and washed twice with saline, care being taken to prepare a suspension free from lumps. In this way the powder was cleared of almost all remaining haemoglobin.