A septic reaction occurring during or following the transfusion of cellular blood components was one of the earliest recognized complications of allogeneic blood transfusions. The presence of bacteria in blood products has been a problem for many decades and currently it is probably the most common microbiological cause of transfusion-associated morbidity and mortality. Transfusion-associated sepsis due to contaminated platelet concentrates appears to be much more common than those due to red cells. The overall prevalence of contaminated cellular blood products (red cells and platelets) is approximately one in 3000; however, the transfusion to a recipient of a contaminated blood product may not necessarily be associated with clinically evident morbidity. This is because the majority of contaminated blood product units contain only few bacteria. In other instances, contaminated units may contain large numbers of virulent bacteria as well as endotoxins, and their transfusion may be associated with significant morbidity and even be lethal to the recipient. The prevalence of severe episodes of transfusion-associated sepsis has not been clearly established, but is probably of the order of one in 50,000 per platelet unit and one in 500,000 per red cell unit transfused. As a result of the increased recognition that such transfusion-associated episodes can occur, a variety of measures have been proposed to try to prevent and/or control the risk of transfusion-associated septic reactions.