Nanoparticles are important industrial materials. However, many nanoparticles show biological effects, including toxic activity. Metal ion release is the most important factor affecting the biological effects of nanoparticles. In addition, nanoparticles have large adsorption ability. The adsorption ability, in particular protein adsorption to nanoparticles, has an effect on cellular uptake and cellular metabolisms. Moreover, the adsorption ability of nanoparticles causes artificial effects in in vitro systems. Consequently, accurate determination of released or secreted proteins such as lactate dehydrogenase and cytokines adsorbed to nanoparticles is affected. In addition, artificial effects cause overestimation or underestimation of the cytotoxicity of nanoparticles. Therefore, measurement of the protein adsorption of nanoparticles is important. Some methods for the determination of the adsorption to nanoparticles have been suggested. The flow field-flow fractionation method is one of the efficient techniques for determining proteins on the surface of nanoparticles. The cellular effects caused by nanoparticles should be carefully considered.