as referring to the treatment of animals is defined herein to mean those quantities of cephalotaxine ester which will cause remission of the leukemia in the animal to which it is administered without imparting a toxic response the effective amount will vary with the particular esters the injection vehicle the strain of leukemia and other related factors generally for the instant esters an effective dose will be in the range of about 1 5 380 mg kg of body weight day the preferred dose range for a given ester is defined by the lowest and highest dose shown in table ii for that ester the activities of these novel cephalotaxyl esters are not predictable and in some cases are totally unexpected from structure activity correlations based upon the naturally occurring active esters and certain other synthetic esters for instance all of the naturally occurring active esters such as harringtonine homoharringtonine and deoxyharringtonine contain a dicarboxylic acyl group and a tertiary hydroxyl group alpha to the carboxyl esterified with the cephalotaxine molecule the hydroxyl group is notably absent in all of our compounds and the trans trans sorbate of example 4 and the trichloroethylcarbonate of example 6 do not exhibit the dicarboxylic acid moiety eighteen other acyl esters synthesized at approximately the same time as the subject acyl esters were inactive for example the cephalotaxyl ester of mandelic acid without the trichloroethylcarbonate moiety appearing in the compound of example 5 had no activity in addition toxicity observed during administration of the.