During the past 10 years, methods for blood component preparation have undergone considerable change. Refinements of traditional methodologies were necessary in response to demands for increasingly effective and safe platelet support for the intensive chemotherapy for patients with malignant disease, as well as a response to evidence of the immunological effects of whole blood or unmanipulated erythrocytes. At the center of the issue is concern for the consequences of transfusing contaminant leukocytes. Clearly, for all transfusions the lowest possible leukocyte content is desirable, and their removal should be as early as possible during storage. This review of recent journal articles focuses on reports of investigations of the effects of allogeneic leukocytes in blood transfusions. There is a sense of competition among the advocates for secondary elimination of leukocytes (by filtration) versus those favoring the approach of primary avoidance of leukocyte contamination (by centrifugation). As the following summaries indicate, the relative merits of leukocyte reduction by filtration, either before or after storage, versus leukocyte reduction by specialized apheresis methods remain undefined. The goal of a universal guideline for the use of leukocyte-reduced blood components is not yet achieved.