5'-Esters of ara-cytidine (1-.beta.-D-arabinofuranosylcytosine) are prepared by reacting ara-cytidine with .beta.,.beta.,.beta.-trihaloethoxycarbonyl halide or other protective agency to form a protective amido group on the primary amino nitrogen of ara-cytidine and then reacting the thus-protected compound with a reagent reactive with the 5'-O-hydroxyl group, e.g., an acylating agent, to form the 5'-O-derivative. The .beta.,.beta.,.beta.-trihaloethoxycarbonyl or other protective group is then removed. Alternately, the primary amino group of ara-cytidine can be protected from acylation by protonation. The 5'-O-derivatives in their free base of salt form are characterized in that they display the property of sustained release of the compound, ara-cytidine, when administered intramuscularly or subcutaneously. Ara-cytidine is known for its anti-viral action and for its usefulness as an agent for controlling leukemias, including acute leukemia, and the sustained release property extends the usefulness of ara-cytidine for these purposes and as an inmunosuppressive agent. The 5'-O-derivartives of this invention can also be administered orally.