Terminal subcultures were performed on 1,162 5-day negative blood culture sets which had been monitored by the Difco ESP 384, a continuous-monitor blood culture system. Of these, 16 (1.4%) had growth upon terminal subculture. The isolates not detected by the Difco ESP 384 were Cryptococcus neoformans (eight isolates), Candida albicans (one isolate), Staphylococcus aureus (two isolates) coagulase-negative staphylococcus (three isolates), Bacillus sp. (one isolate), and Corynebacterium sp. (one isolate). Acridine orange staining was performed on 200 randomly selected negative blood culture sets from the study group. Of these, two sets were positive and grew out C. neoformans, as did the terminal subculture. A review of patient's medical records indicated that many of these false-negative isolates were clinically insignificant. The Difco ESP 384 failed to detect 1.4% of the isolates in this study, 50% of which were C. neoformans, indicating a deficiency in the detection mechanism of the system. Further studies demonstrated that while these isolates (C. neoformans) grew in the Difco media, the system did not detect this growth when the standard 5-day protocol was used.