A non-linear three-compartment model is proposed to describe a new strategy for the administration of 2',3'-dideoxycytidine (ddCyd) in the treatment of HIV infections. The drug is injected after having been encapsulated in a non-diffusible form (ddCMP) into erythrocytes. Numerical solutions show that by this treatment the highest ddCyd blood concentration is strongly reduced and in turn its toxicity, while long-lasting therapeutic effect is assured. The model is compared with experimental data in vitro.