Wave phase conjugation of ultrasound scattered by clouds of micro-bubbles in water has been studied experimentally and expounded theoretically. The clouds of microbubbles with variable concentration and sizes have been generated here using electrolytic method. The wave front of the ultrasound beam of frequency 10 MHz was reversed by a parametric phase conjugator. The signal of phase conjugate wave (PCW) detected by an acoustic transceiver was compared with the signal of the wave scattered toward the phase conjugator. The scattered wave (SW) signal was detected by the transducer substituting the phase conjugator. It is shown that, in contrast with stochastic SW signal, wave phase conjugation forms regular PCW signal on the transceiver in spite of random distribution of the scatterers. The PCW signal is found to be much more sensitive to variations of bubbles concentration comparing with the mean value of the SW amplitude. Moreover, the relative error of measurements of PCW signals is much smaller than that of the SW signal. The revealed properties of phase conjugate waves are applicable for testing of concentration of scatterers in dispersive systems.