A method of detecting Escherichia coli (EC) in in vivo blood with septicemia was sought using a handmade macro three-electrode (ME) system. Towards this end, an analytical ME probe was prepared using mercury immobilized on a carbon nanotube working electrode, and two graphite pencils were used as counter and reference electrodes instead of the expensive Ag/AgCl standard and Pt counter electrode. Under the optimum conditions, the cyclic potential was obtained at a 0.05 V anodic, then the square-wave stripping working range was obtained at 0.5-3.5 ml (3x102~4x102CFU/1ml) EC. Also under the optimum conditions, the statistic stability was examined in 1 ml EC 15th repeat anodic, with a relative standard deviation of RSD=0.98x10-2. The developed method was applied to non treated blood, using only one drop. The patient blood with septicemia had a fast accumulation time (30 sec). A diagnostic detection limit of 0.06 ml EC was obtained. The recognition methods can be applied to in vivo vascular blood and real organic fluids, and can be used for poisoning control.