Microprobe X-ray absorption near edge structure (mu-XANES) measurements were used to determine directly, for the first time, the oxidation state of intracellular plutonium in individual 0.1-mu m(2) areas within single rat pheochromocytoma cells (PC 12). The living cells were incubated in vitro for 3 h in the presence of Pu added to the media in different oxidation states (Pu(III), Pu(IV), and Pu(VI)) and in different chemical forms. Regardless of the initial oxidation state or chemical form of Pu presented to the cells, the XANES spectra of the intracellular Pu deposits were always consistent with tetravalent Pu even though the intracellular milieu is generally reducing.