From January 1972 to July 1974, 28 patients with bone marrow depression due to aplastic anemia or cytostatic treatment, were transfused with packed cells and platelet concentrates, both containing 10-20% of the amount of leukocytes present in whole blood. Of these patients, 26 (93%) became refractory to random platelets. Since July 1974, 68 patients have been given red cells filtered through cotton-wool, a procedure which removes over 97% of the leukocytes, and leukocyte-poor platelet suspensions obtained by an additional centrifugation step. Of this latter group, 16 patients (24%) became refractory. Fifty-two recipients were non-refractory to random platelet transfusions after an exposure time of at least 6 weeks and maximally 32 weeks. A possible explanation is that platelets are less immunogenic than leukocytes; on the other hand platelets may not be immunogenic at all with regard to induction of HLA antibodies, and the occurrence of the immunization is purely the result of the contamination with leukocytes in the red cell and platelet preparations.