Bactec NR-660, a computerised blood culture system using infrared analysis of microbial generated carbon dioxide, was compared with the Signal system, which detects gaseous pressure (due to bacterial metabolism) by a manometer. Four trials were undertaken: an in vitro evaluation of 99 bacteria in simulated blood cultures, and three prospective comparisons of a total of 2588 paired patient samples. Combined results for bacteria in simulated blood cultures showed a highly significant difference (p less than 0.001) between Bactec NR-660 aerobic medium (6A) and any other phial under test. Detection rates for most bacteria by Signal were on average three times slower than the first Bactec phial (mean delay 58.3 hours). Overall, the systems were not comparable.