Bactec medium 9240 (Becton Dickinson, MD, USA) is a blood culture medium used routinely at Sultan Qaboos University Hospital (SQUH). The medium is said to contain substances that can neutralise antibiotics and destroy leucocytes in blood samples. In this study, the ability of the medium to neutralise the effect of some antibiotics and to destroy leucocytes is investigated. Vancomycin, amoxicillin, chloramphenicol, penicillin, gentamicin, fungizone and amikacin at various concentrations were added to separate bottles of Bactec medium 9240. Escherichia coli (NCTC 10418), Pseudomonas aeruginosa (NCTC 10662), Staphylococcus aureus (NCTC 6571) and Candida albicans (ATCC 10231), each at 1 x 10(4) colony-forming units (CFU)/mL were added separately, depending on the type of antibiotic. The blood samples were incubated at 37 degrees C for seven days under manual and automated blood culture systems. Subcultures were made from the manual system and routine laboratory procedures for detection of positive cultures were followed for the automated system. The Bactec medium was found to neutralise all antibiotics up to a concentration of 100 microg/mL by the automated method but showed some variation in results by the manual system. Leucocytes were destroyed within 24 hours.