We have followed the evolution of leucocyte antibodies in patients receiving multiple blood transfusions of leucocyte-poor blood. The number of leucocytes present in one unit of transfused blood was always less than 1 X 10(8). We observed that such preparations were able to re-stimulate antibodies which had disappeared, but especially they were able to provoke a first immunisation in some patients never immunized in the past. The specificities of these antibodies corresponded to one or several antigens present in the donor of the transfused unit. This study shows that leucocyte immunisation cannot always be avoided in spite of the selection of blood products containing very few white cells.