The disposition of 14C-labelled and 99mTc-labelled APD has been investigated in the Balb/c mouse following a single ip injection containing APD (total dose 5 mg/kg) labelled partly with carbon-14 and partly with technetium-99m. The plasma clearance of carbon-14 was twice that for technetium-99m and the accumulation of carbon-14 in bone was twice as great as that of technetium-99m. All other tissues assayed (liver, lung, kidney and muscle) contained higher concentrations of technetium-99m than carbon-14. There was a particularly marked accumulation of technetium-99m in the liver. We conclude that pharmacokinetic studies of diphosphonates cannot be conducted reliably with 99mTc-labelled compounds.