The effectiveness of red blood cells made leukocyte-free by filtration through cotton wool to prevent the production of antileukocyte antibodies was evaluated in children suffering from Cooley’s anemia. Two studies were performed: study I was carried out prospectively in two groups of non transfused patients, one group treated with leukocyte-free filtered red cells, the other with buffy-coat-free packed red cell units. Different types of antileukocyte antibodies were looked for in both groups and the results were compared. In study II the behavior of pre-existing lymphocytotoxic antibodies found in the serum of children previously transfused with standard or buffy-coat-free packed red cell units was followed after the patients had been passed to a program of transfusion with leukocyte-free filtered red cells. Study I showed that none of the patients transfused with leukocyte-free filtered red cell units have produced antileukocyte antibodies, while these could be found in 2/3 of the patients transfused with buffy-coat-free packed red cell units. Study II showed that the repeated transfusion of leukocyte-free filtered red cells to patients who possessed in their serum preformed lymphocytotoxic antibodies did not cause any increase in the potency or spectrum of these antibodies, but was in fact accompanied in some cases by their decrease or disappearance. It is concluded that filtration through cotton wool is an easy and inexpensive means of preparing leukocyte-free red blood cells for transfusion capable of preventing (or reducing) the production of antileukocyte antibodies in multitransfused patients.