Abstract [5′-3 H]-5-Chloro-2′, 3′-diodeoxy-3′-fluorouridine (1; R=[3 H]) was prepared at a specific activity of 10.2 Ci/mmol suitable for development of a radioimmunoassay procedure. The synthetic sequence employed controlled oxidation of unlabelled 1 to the 5′- aldehyde (2), isolation as the imidazolidine adduct (3), regeneration of the free aldehyde, reduction with [3 H] NaBH 4, and purification by preparative TLC. The radiochemical purity ...