The sensitivity and time to detection of Brucella melitensis by the BACTEC 9240 and the Isolator blood culture systems were compared in a prospective volume-controlled study. Blood sample aliquots, obtained from children with suspected brucellosis, were inoculated into a BACTEC 9240 Peds Plus bottle and into an Isolator 1.5 Microbial Tube. Overall, 122 pairs of blood samples for culture were obtained, and 28 (23%) were positive by at least one method. The BACTEC 9240 system detected all 28 positive cultures (sensitivity, 100%), and the Isolator system detected 22 positive cultures (sensitivity, 79%) (P = 0.023). Among those 22 cultures positive by both methods, 21 (95%) and 15 (68%) were found to be positive within 3 days by the BACTEC and the Isolator systems, respectively; 8 (36%) were found to be positive at least 1 day earlier by the BACTEC instrument, and the remaining 14 were found to be positive by the two systems on the same day (P = 0.045). The BACTEC 9240 blood culture system is more sensitive than the Isolator system for the detection of B. melitensis and is superior in terms of time to detection of the organism.