Platelet concentrates were contaminated with predetermined numbers of bacteria isolated from patients. Staphylococcus epidermidis and Pseudomonas aeruginosa organisms were killed if the number introduced was less than 10(3). When 10(4) through 10(6) organisms were added, their growth was depressed for 24 hours and then slowly resumed. If more than 10(7) organisms were used, a slow and progressive growth occurred. After the concentrates were contaminated with Staphylocoecus aureus, slow progressive growth occurred regardless of the size of the inoculum. Cultures taken from sample phlebotomies performed on a few donors showed that the number and type of organisms released into the blood during the venepuncture are such that they could usually, but not always, be inactivated by the bacteriocidal property of the platelet packs. This bacteriocidal property will usually allow the platelet packs to be stored at room temperature for considerable amounts of time with relative safety. Bacterial growth will normally occur only when the number of organisms introduced during the venepuncture is quite large, and growth will usually occur during the first 24 hours.