The possible beneficial role of white cells (WBCs) in donor blood has been investigated with respect to their capacity to remove bacteria. Preparations of buffy coat and whole blood, containing as well as reduced of WBCs, were inoculated with Staphylococcus epidermidis, S. aureus, Escherichia coli, Pseudomonas aeruginosa, and Propionibacterium species. Upon storage at room temperature, the presence of WBCs resulted in a reduction of the bacterial content. Units inoculated with S. epidermidis and E. coli were completely cleared of bacteria within 5 to 24 hours. On the other hand, S. aureus, after an initial reduction in number, started to multiply. In WBC-reduced units, the initial bacterial content remained unchanged for 5 hours, but the bacteria then exhibited vigorous growth within 48 hours in buffy coat and slower growth in whole blood. Propionibacterium sp. did not grow with or without WBCs. P. aeruginosa did not grow in buffy coat but showed a growth pattern similar to that of S. aureus in whole blood. The presence of WBCs in the donor blood during the first hours after collection thus seems to rid the blood of at least some species of bacteria. These results indicate that it would be favorable not to perform WBC reduction during blood collection and that several hours of contact can be needed to obtain sterility.