Cyclic imides such as N-substituted alkyl ethers, thioethers, sulfoxides, sulfones and related derivatives were potent agents against human single cell tumors and selected solid tumor growths, eg adenocarcinoma of the colon and glloma. These agents in the L1210 lymphold leukemia tumor model preferentially Inhibited DNA synthesis. The regulatory enzyme sites in the purine pathway were targets of the agents. Other sites of inhibition were DNA polymerase α and thymidylate synthetase activities. d(NTP) pool levels were also reduced by the agents over 60 min.