The authors designed an elemental analysis system using an ion microbeam combined with a microparticle-induced X-ray emission (micro-PIXE) method for the analysis of biomedical samples in air with a spatial resolution of 1 µm (in-air micro-PIXE system). This system was used to develop an imaging and quantification method for intracellular cis-diamminedichloro-platinum(II) (CDDP) in a human lung cancer cell line. A human lung adenocarcinoma cell line, A549, was cultured and nuclear labeling was carried out by incubating the cells with BrdU. The cells were then exposed to CDDP at concentrations ranging from 1 µmol to 1 mmol, for 30 min to 24 h. After drug treatment, samples were washed and frozen with liquid nitrogen, and freeze-dried for 24 h. Standard samples were made using agar containing several concentrations of CDDP. Experiments using standard samples showed a linear correlation between CDDP concentration and platinum signal strength. No clear platinum signal was detected after exposure to CDDP for 24 h at doses between 1 and 100 µmol. However, significant platinum signals were observed at 1 mmol. When nucleus and cytoplasm visualization was sufficiently clear to efficiently use in-air micro-PIXE, the platinum image quality was considered satisfactory. The detected signals of CDDP were stronger in the nucleus than in the cytoplasm. A time-course study showed increased CDDP uptake in cells after longer drug exposure periods. The present study demonstrates the application of element analysis using in-air micro-PIXE to biomedical samples. The use of this system enables the high-resolution visualization of intracellular CDDP distribution and measurement of intracellular CDDP concentrations. (Cancer Sci 2008; 99: 901–904)