Current trends in the search for chemical compounds having an inhibitory action on the growth of malignant cells are reviewed in this article. Several agents are sufficiently promising that clinical trials with them are in progress. One of these, an aromatic nitrogen mustard (C.B. 1348), appears to be useful as an adjunctive therapeutic measure in patients with malignant lymphoma, chronic lymphocytic leukemia, and mycosis fungoides who have become refractory to other methods of treatment. The introduction of certain purine antagonists, of which 6-mercaptopurine has been given the most extensive clinical trial, has opened up a new field of experimental and clinical investigation. 6-mercaptopurine and related compounds appear to be particularly useful in the treatment of acute leukemia in adults, but they are also useful, together with the folic acid antagonists and the steroid hormones, in the management of acute leukemia in children. While at present chemotherapeutic agents currently under investigation rarely cause significant regression of inoperable primary or metastatic solid tumors, the possibility of eventual more effective control in many types of malignant disease is not as dismal as it was a decade ago.