The morphological and biochemical changes that occur after inoculation of sterile blood into a blood culture medium (tryptic soy broth) with sodium polyanetholesulfonate and CO 2 were investigated. Cellular changes, pH, PCO 2 , and PO 2 were monitored and evaluated. Erythrocytes became crenated and developed precipitated hemoglobin inclusions within 4 h. The lymphocytes appeared morphologically intact at 24 h, and by 48 h a few cells had undergone transformation. Many neutrophils were vacuolated at 24 h. Neutrophils capable of phagocytizing Staphylococcus aureus were observed after 18 h of incubation. Identifiable eosinophiles were present on day 6 of the study. A decrease in PO 2 in the unvented bottles from 44.4 to 8 mm of Hg occurred by 24 h. PO 2 remained low for 6 days, after which a slight increase occurred. An increase in PO 2 in the vented bottle from 51 to 58 mm of Hg occurred by 24 h of incubation. In both the vented and unvented bottles the PCO 2 increased. This increase was markedly more rapid in the unvented bottle. From a pH of 7.06 a decrease occurred for the first 24 h after inoculation, with the pH stabilizing at 6.8 in the vented bottles and at 6.6 in the unvented bottles. The biochemical changes that occurred in the vented culture bottles stabilized more rapidly than those of the unvented bottles. Changes caused by the addition of sterile blood to a blood culture medium resulted in conditions which departed considerably from accepted optima for the isolation of clinically important microorganisms. The phagocytosis of organisms that occurred may also have reduced the yield.