Detection limit of any method is affected by device (electronics, noise), evaluation, software (timing inaccuracy, rounding mistakes, errors determination of parameters using measured values by numerical methods) and by a chemical reaction (diluting errors, the reaction mixture design, chemical interferents, temperature, the accuracy of pipetting). In this work was proposed a metod for determination of detection limit for measuring of inhibitors of acetylcholine esterase by biosensor analysator of toxicity (BTA). The analysator was developed by BVT Technologies a.s. in 2004. To determine the exact detection limit a calibration of output signal was done. Presently the analysator is able to detect only presence of toxins, not their concentration. Not least are mentioned all founded errors of software, which are continuously eliminated.