Combination effect of antitumor agents, including cyclocytidine and cytosine arabinoside, was evaluated on the conception of pharmacological synergism and not of therapeutic synergism. Ascites sarcoma-180 and L-1210 leukemia were used as tumor systems. In sarcoma-180 system, combinations of cyclophosphamide plus cyclocytidine or cytosine arabinoside by simultaneous administration and cyclocytidine plus Daunorubicin or Vinblastine by alternate administration provided synergism. In L-1210 system, many compounds in combination with cyclocytidine or cytosine arabinoside in both simultaneous and alternate administrations provided synergism. Combination effect of agents was affected by the schedule of drug administration. It was found that the combination effect of drugs in one tumor system cannot be generalized to that in other tumor systems, even though equally effective doses of agents were administered in both tumor systems. Toxicity of cytosine arabinoside in combination with other drugs was affected by the schedule of administration. Compounds which provided synergism in simultaneous administration provided antagonism in alternate one. As a result, it was found that alternate administration of drugs is advantageous for the activity and diminution of toxicity of the host animal.