A method and apparatus for therapeutically treating cancer is provided. The apparatus includes a magnetic field generator for producing a controlled, fluctuating, directionally oriented magnetic field parallel to a predetermined axis projecting through a malignant neoplasm. In one aspect, a field detector measures the magnetic flux density along the predetermined axis. The applied magnetic field may comprise a full-wave rectified signal oscillated at predetermined frequencies to maintain a preselected ratio of frequency to the effective flux density, where the ratio regulates the growth characteristics of cancer cells of the neoplasm. This ratio is maintained by adjusting the frequency of the fluctuating magnetic field and/or by adjusting the intensity of the applied magnetic field after nulling out the local magnetic field at that region containing the neoplasm. In one aspect, a synergistic therapeutic cancer treatment is obtained by exposing cancer cells to the magnetic fields of the invention in the presence of a chemotherapeutic cancer agent.