A linear-scaling method to carry out semiempirical quantum mechanical calculations for large systems has been developed based on the density matrix version of the divide-and-conquer approach. The method has been tested and demonstrated to be accurate and efficient. With this implementation, semiempirical quantum mechanical calculations are made possible for large molecules over 9000 atoms on a typical workstation. For biological macromolecules, solvent effects are included with a dielectric continuum model.