Abstract: By using protein binding assay for cAMP it was shown that, in contrast to the spleen of the inbred 615 mice; very low cAMP contents in the spleen of mice bearing leukemia L615 were observed. At the dosage of 0.9 mg/kg, harringtonine caused marked increase of cAMP levels in the spleen of leukemia L615 bearing mice. isoharringtonine, homoharringtonine, deoxyharringtonine and semisynthetic harringtonine showed similar effects in raising cAMP levels of the leukemic cells, among which the effect of deoxyharringtonine was most significant. A close correlation between increased cAMP content in spleen and therapeutic effects against leukemia L615' was also noted.No significant changes were observed in cAMP content in the P388 leukemic cells treated with these esters of cephalotaxine.