An analysis was performed of the microbiological laboratory quality control data for the past 9 years (1985-1993). Bacterial contamination was detected in 100 of 25,171 tested blood components. Single-donor platelet concentrates had a contamination incidence of 25 in 5889 (0.42%); whole blood samples were contaminated at a rate of 1 in 2973 (0.03%) and red cell concentrates at a rate of 73 in 15,317 (0.48%); of 992 samples of fresh frozen plasma only 1 was contaminated (0.1%). Gram-positive staphylococci comprised 75% of cases and Gram-negative rods 10%. This frequency of laboratory-detected bacterial contamination contrasts with the low rate of transfusion-associated septicaemic events.