For 14 months, effects of volume on blood culture with Bactec 9240 are studied. Amongst the 1392 vials sending a signal out, 15.5% are false positive (FP). They stand out as such true positives by that very fact: negative subcultures, higher blood volumes, shorter time for detection, greater number of aerobic vials. Exclusively for FP, blood volume and detection delay are linked. In short, whatever the volume of culture may be, the prevalence of aerobic vial is observed with FP. The best volume of blood for the positive vials is 8 ml. Blood volume for each vial must be distinguished from whole culture volume of blood for each patient. If Bactec 9240 seems to be perfectible, for now, its optimum use should be septic discharge culture of about 30 blood milliliters shared out amongst 2 aerobic vials and 2 anaerobic vials, and incubation time should be kept at 7 days.