The toxicity and the internalization, adhesion, and dispersion behavior of manufactured polystyrene latex (PSL) nanoparticles (nominal diameter: 50 nm) with various functional groups toward the yeast Saccharomyces cerevisiae (which was applied as a model eukaryote) were examined using the colony count method, and microscopic observations. The colony count tests suggested that PSL nanoparticles with a negative surface charge showed little or no toxicity toward the yeast. In contrast, PSL nanoparticles with an amine functional group and a positive surface charge (p-Amine) displayed a high toxicity in 5 mM NaCl. However, the yeast cells were mostly unharmed by the p-Amine in 154 mM NaCl, results that were quite different from the toxicological effects observed when Escherichia coli was used as a model prokaryote. Confocal and atomic force microscopies indicated that in 5 mM NaCl, the p-Amine nanoparticles entirely covered the surface of the yeast, and cell death occurred; in contrast, in 154 mM NaCl, the p-Amine nanoparticles were internalized via endocytosis, and cell death did not occur. (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan.