A series of adenosine 3', 5'-cyclic monophosphoramidates (3, cAMP amidates), including long-chain alkyl amidates, were synthesized from adenosine 3', 5'-cyclic monophosphate (1, cAMP) by means of a one-pot reaction. THis reaction proceeded by the treatment of cAMP tributylammonium salt (2) with phosphorus pentachloride (PCl₅) and alkylamine in N, N-dimethylformamide (DMF). Compounds 3 synthesized were investigated to determine their cytotoxic activities on the growth of mouse mastocytoma P-815 cells, mouse manamary tumor FM3A cells, and human mammary tumor ZR-75 cells in culture. It was found that compounds 3h-m showed significant cytotoxic activities against these cell lines, and that cAMP decylamidate (3j) was the most cytotoxic compounds (the concentration required for 50% inhibition of cell growth, ID₅₀=6.0, 15.0, 2.2 μM, respectively); the antitumor effect on P-815 cells by a total packed cell volume method showed 81.8% inhibition. The cytotoxic activity of 3 increaesd with the increase in alkyl chain length up to 10 carbon atoms and decreased in compounds having longer alkyl chain.