Background of a well-type ground-based low-level high purity germanium spectrometer was reduced using a single top side anticoincidence proportional chamber. The chamber suppressed the total background (40–1500 keV) of 34 cpm by a factor of 2.2. To assess the efficiency of the anticoincidence chamber in reducing the cosmic muon induced background Monte Carlo simulations were done using GEANT3. With the sea-level cosmic muon flux distribution, a total muon induced background of 47 cpm (0–1500 keV) and a background reduction factor of 4.2 were computed. Good agreement between the measured and simulated spectra was obtained. Some general features of the background induction by muons are discussed. Muons with momenta from about 1 to 10 GeV/ c contribute mostly to the background. For muons with momenta below 1 GeV/ c the muon decay is the most contributing mechanism.