The increasing use of nanoparticles makes it necessary to check up possible toxicological risks of this new materials class. In this paper we describe on two nanopowders (tungsten carbide, titanium nitride) which methods and parameters of a chemical-physical characterization are needed in forefront of toxicological experiments. This includes investigation on the powder itself as well as on particles suspended in water and physiological media, respectively. The most important result is that nanoparticles agglomerate in serum-free medium within minutes, whereas in the present of serum an agglomeration is inhibited. Hence, we have physiological suspensions with well-distributed stabilized particles which allow performing toxicological testing under reproducible conditions. Furthermore we could prove that tungsten carbide particles were taken up into cells, but no acute toxicity was found determined by means of in vitro viability tests with different cells.