3'-[3-(3-Azido-2,3-dideoxy-β-D-erythro-pentofuranosyl)-3,6-dihydro-5-methyl-2,6-dioxo-1(2H)-pyrimidinyl]-3'-deoxy-thymidine-d3 is an isotope labelled analog of 3'-[3-(3-Azido-2,3-dideoxy-β-D-erythro-pentofuranosyl)-3,6-dihydro-5-methyl-2,6-dioxo-1(2H)-pyrimidinyl]-3'-deoxy-thymidine (A825005). 3'-[3-(3-Azido-2,3-dideoxy-β-D-erythro-pentofuranosyl)-3,6-dihydro-5-methyl-2,6-dioxo-1(2H)-pyrimidinyl]-3'-deoxy-thymidine is used in the biological studies and in the preparation of N-3-substituted thymidine nucleosides as virucides as it has anti-HIV activity.