THE principal aim in cancer chemotherapy is to find compounds which kill the tumour cells without irreversibly damaging normal cells of the host. If the cells of a cancer, like many normal cells of the body, are continuously dying, the cancer could be controlled by slowing the rate of tumour cell proliferation below the rate of tumour cell death. Tumour remissions could then be obtained by administration of a drug which does not directly kill the tumour cells. One of the most successful chemotherapeutic agents, amethopterin (methotrexate), may act in this way.