The kinetics of 3′-azido-3′-deoxythymidine phosphorylation with [32P]orthophosphoric acid was studied in the presence of various coupling agents. The most effective method with the use of BrCN provided the isolation of the target 3′-azido-3′-deoxythymidine 5′-[ 32P]monophosphate in 46% yield and a high specific radioactivity (100 Ci/mmol).