Bactec Plus blood culture bottles were preincubated at 35 degrees C or at room temperature before entry into the Bactec 9240 instrument to determine the influence of preincubation temperature and time. Of 463 positive blood culture sets, 956 bottles were positive, of which the instrument detected 92.1%. Of 76 positive bottles undetected by the instrument, 68 were preincubated at 35 degrees C and eight at room temperature. The median entry delay and instrument detection times were 17.9 and 7.2 h for preincubated bottles, and 16.4 and 13.4 h for bottles held at room temperature. Short entry delay and inspection before entry into the instrument are necessary if preincubation at 35 degrees C is used.