A new Azidothymidine derivative, di-(thymidine-3'- azido-2',3'-dideoxy-D-riboside)-5'-5'-p1-p2-pyrophosphate (AZTp2AZT), was encapsulated in human erythrocytes according to a conservative procedure of hypotonic shock-isotonic resealing and reannealing. Like in erythrocyte lysates supplemented with 1 mM ATP, intact red cells too were found to convert AZTp2AZT to 3'-Azido-3'-deoxythymidine which was then released linearly in plasma. The major metabolic pathway involved in this conversion was the symmetrical hydrolysis of AZTp2AZT to yield two 3'-Azido-3'- deoxythymidine-5'-phosphate molecules which were then dephosphorylated to 3'-Azido-3'-deoxythymidine. At late times of incubation, also a limited asymmetrical hydrolysis of AZTp2AZT became apparent in the intact erythrocytes, yielding 3'-Azido-3'-deoxythymidine-5'-diphosphate that was then converted to the triphosphorylated derivative. Therefore, erythrocytes loaded with AZTp2AZT act "in vitro" as bioreactors ensuring sustained and potentially useful release of 3'-Azido-3'-deoxythymidine.