The usefulness of the nitro blue tetrazolium (NBT) test as an aid in the detection of systemic bacterial infections was evaluated prospectively in 32 children who had undergone open-heart surgery. After the first postoperative week, the NBT test was found to be a valuable screening procedure. The percentage and absolute number of NBT-positive neutrophils were substantially elevated in five patients who subsequently were found to have bacterial endocarditis or pneumonia. Children with wound infections, nonbacterial febrile illnesses, and postcardiotomy syndromes had normal NBT test results. "Falsely" elevated NBT test values occurred during the first week following surgery in ten patients who subsequently did not develop bacterial infections. The reasons for the unreliability of the NBT test during this period are unclear.