A cell array has been developed for single cell analysis with magnetic cell labeling method using the unmodified colloidal magnetic nanoparticles (magnetite particle with 10 nm diameter). When the magnetic nanoparticles were dispersed in the medium for cell labeling, these particles aggregate to micrometer order sizes. However, when these particles were dispersed in a sucrose solution, above 80% of these particles maintained their size below 100 nm and their dispersibility was kept for at least two weeks. When mouse pulmonic fibroblasts L929 were exposed to the magnetic nanoparticle suspension of a 0.3 M isotonic sucrose solution for 10 min, it was found that the magnetic nanoparticles were taken into the cells at approximately 1.1 pg-Fe/Cell This method was demonstrated without modifying the magnetic nanoparticles, and a variety of cells could be labeled magnetically. It was found that this magnetic labeling method did not influence the cell response to cytokine, TNF-alpha. The magnetic induction by means of a pin holder device, which is made from magnetic soft iron and contains more than 6000 pillars on its surface, enabled the arrangement of the magnetically labeled cells in arrays. In addition, the method was used to monitor the time course of the responses of L929 cells to TNF-alpha in a single cell array. The change in the stages of apoptosis such as phosphatidylserine externalization and propidium iodide uptake was detected using fluorescent microscopy.