The effect of entry delayed blood culture bottles until the start of incubation for mechanical detection of organism were compared using 2 major blood culture systems; BACTEC 9240 system and BacT/ALERT 3D system. Total of 13 bacterial strains; 5 gram-positive cocci, 7 gram-negative bacilli and Candida parapsilosis which were isolated mainly from blood cultures were used as the test strains. BACTEC 92F, 93F and BacT/ALERT FA, FN bottles were used as the blood culture bottles. All the bottles inoculated with the test strains were incubated and evaluated immediately after standing at room temperature for 24, 42, 48, 54 or 72 hours, using the respective automated blood culture systems. All the bottles were subcultured. The effect of entry delay the blood culture bottles for the mechanical detection was observed in many gram-negative organisms in BACTEC 9240 system. The blood cultures were evaluated not to be positive in 4 of the 10 samples on delaying for 24 hours or in any of the samples on delaying for 42 hours in the BACTEC 92F bottles inoculated with Escherichia coli. In Serratia marcescens, the blood cultures were evaluated not to be positive in 5 of the 10 samples on delaying for 24 hours or in any of the samples on delaying for 42 hours in the BACTEC 92F bottles. In Klebsiella pneumoniae, the blood cultures were evaluated not to be positive in 9 of the 10 samples on delaying for 42 hours. In Enterococcus faecalis, Pseudomonas aeruginosa and Proteus mirabilis, the blood cultures were evaluated not to be positive in 5-6 of the 10 samples on delaying for 42 hours. On the other hand, the blood cultures were evaluated to be positive in most of the samples of Acinetobacter calcoaceticus (except 3 of the 10 samples which were evaluated not to be positive) on delaying for 42 hours in BacT/ALERT 3 D system. The samples except part of Streptococcus spp. were detected by subculture in both the bottles. These results indicate that the delayed time of blood culture bottles before inoculation with the test bacterial samples affects the positive detection of blood cultures markedly in the blood culture system. Therefore, the immediate incubation was considered to be necessary.