The distribution and retention of 227Ac in rat tissues has been studied after intravenous injection of the nuclide, in equilibrium with its daughters, as a complex with serum proteins, as nitrate, citrate or DTPA complex. When 227Ac was administered as a serum protein complex, about 40% of the injected dose was excreted within 28 days and the remainder was lost with a half time of about 700 days. Except when complexed with DTPA, the tissue distribution oμ 227Ac was not markedly influenced by the chemical form of the injected material. Treatment with DTPA or TTHA caused a 60% reduction in the body burden of 227Ac, but only when treatment was started within 30 min of exposure. Studies of the distribution of 227Ac amongst the serum proteins suggest that transferrin may be one of the proteins concerned in the transport oμ actinium in the blood. Subcellular fractionation of liver has shown that 227Ac is associated with the lysosomes at 4 days after injection.