The BACTEC 225 was used to test 5,811 routine blood cultures over a 20-month period. Aerobic, anaerobic, and hypertonic media were employed. The BACTEC 225 detected 511 positive cultures; 407 of these were considered significant organisms, and 104 were presumed contaminants. Of the significant positive cultures, 15% were detected within the first 12 h of incubation, 52% within 24 h, 82% within 48 h and 92% within 72 h. Aerobic, anaerobic, and hypertonic media are recommended for each venipuncture since 56 cultures were isolated from the aerobic medium only, 110 from the anaerobic medium only, and 94 from the hypertonic medium only. There were 16 patients who had multiple venipunctures from which organisms were repeatedly isolated from only one medium: two from the aerobic medium, four from the anaerobic medium, and ten from the hypertonic medium only. Detection times were not significantly different for the aerobic and hypertonic media. However, there were five patients with multiple venipunctures in which growth was detected radiometrically at least 48 h earlier in the hypertonic than in the aerobic medium. False-positive growth index readings were noted in 1,085 (19%) of the aerobic vial, 11 (0.19%) of the anaerobic vials, microorganisms were isolated from at least one of the companion vials. Using 5% co2 to flush the aerobic vials. With some false-positive aerobic and hypertonic vials decreased the number of false positives to about 6% of the total.