X-ray crystallographic studies will be undertaken on three classes of compounds which are closely tied to the problems of cancer causes and treatment. Two of these types are drugs which are very useful agents currently in use in cancer chemotherapy: cyclophosphamide derivatives and substrates and inhibitors of dihydrofolate reductase. The compounds which make up the third type that we will study are di- and poly- deoxyribonucleotides, the molecular subunits of DNA. The purpose of these studies on the cancer chemotherapeutic agents will be to elucidate the stereochemical parameters for these compounds and to compare the stereochemistry of active and inactive drugs to determine conformational requirements for anti-tumor activity.