Abstract : Laser cooled atoms have been used as a nonlinear medium to reduce quantum fluctuations of a laser beam. The atomic sample is produced by a magneto-optical trap and placed inside an optical cavity where it is interacting with the probe beam. When the trap is switched off, the atomic cloud undergoes a ballistic expansion, an effect which is used to scan the cavity across resonance. This system exhibits bistable behavior and noise measurements can be performed during a time slot of 20-30ms. A quadrature squeezing of the order of 40 ± 10% due to the nonlinear interaction has been observed under such conditions. On the theoretical side, a treatment of the variation of mean number of atoms in the probe beam as well as of atomic number fluctuations has been developed. A full treatment of quantum fluctuations of the probe beam has been performed taking also into account its transverse mode structure. A synthesis of the theoretical developments has been used to model the experimental noise spectra. The theoretical spectra obtained are in satisfactory agreement with the experimental observations. A quantum noise reduction has also been observed in presence of the magneto-optical trap with very weakly trapped atoms. The best measured squeezing under these conditions is of the order of 20 ± 10%.