Bacterial infections transmitted by blood or blood products, although rare, remain a serious threat to the recipient of a transfusion. We report on five cases of adverse reactions due to bacterial contamination of blood products, and we review 76 similar cases reported in the English-language literature. Most cases (70%) have been reported from the United States. Various sources of contamination have been suggested, including infection in the donor and invasion of the blood product during the process of collection, preparation, and storage. Frequent clinical manifestations are fever (80%), chills (53%), hypotension (37%), and nausea or vomiting (26%). The overall mortality is 35% (28 of 81 patients). In 38 patients (47%) the adverse reactions have appeared during transfusion; in the others the interval between completion of the transfusion and appearance of symptoms has ranged from 15 minutes to 17 days. A wide spectrum of bacteria have been implicated as causes of adverse reactions, with Pseudomonas species involved in 28% of episodes. Many such reactions are probably misdiagnosed or overlooked, the result being underestimation of the extent of the problem.