A new highly sensitive method of detection of the Browns or bio-particles in liquids, based on the fluctuations intensity spectroscopy, is described. A special scheme of disposition of photodetectors has made it possible to significantly reduce noise due to intensity fluctuations of probe laser radiation. As a result the detection sensitivity increases and is mainly restricted by electronical noise. We demonstrate the high sensitivity of the method allowed to detect the presence of viable micro-organisms in natural water reservoirs. The optical scheme of the device and preliminary experimental data are presented.