The aim of this thesis was to establish syntheses of Ag-Au-nanoparticles in aqueous media with subsequent characterization of the nanoparticles as well as their cytotoxicity. Based on the prominent Turkevich-method and comparable syntheses of Ag-Au-nanoparticles, citrate-stabilized spherical Ag-Au-nanoparticles were prepared in nine different compositions by co-reduction of suitable silver- and gold-precursors with tri-sodium citrate. Due to a lack of stability the particles were functionalized with poly(vinylpyrrolidone), which furthermore made a purification of the nanoparticles by ultracentrifugation possible. The experimental Ag:Au compositions were examined by AAS and showed good agreement with the theoretical values. Based on UV-Vis-analyses of the surface plasmon resonance of the particles, a high degree of alloying was determined. Due to significant differences in the size of the citrate-stabilized nanoparticles with different compositions, polydispersity and poor stability, a stronger reducing agent, i.e. a mixture of tri-sodium citrate and tannic acid, was used to modify the synthesis. The reduction of Au3+ and Ag+ with a mixture of citrate and tannic acid led to spherical Ag-Au-nanoparticles with improved monodispersity and stability which could be purified by ultracentrifugation. The theoretical Ag:Au-composition was verified by AAS. Compared to the citrate-stabilized particles, the citrate/tannin-stabilized particles were smaller but showed a gradual trend in particle size that was dependent on the silver content. A functionalization of the Ag-Au-nanoparticles with PVP and TPPTS was carried out and verified by means of colloidal and spectroscopical analyses. Although a high degree of alloying was detected by UV-Vis spectroscopy, HR-TEM (HAADF, STEM) showed that the distribution of silver and gold inside individual ~12 nm Ag:Au-50:50-particles was not statistical. For the citrate/tannin mixture the influence of the amount of reducing agent on the resulting particle size was examined. As it was expected, a lesser quantity of reducing agent led to larger nanoparticles, while an increase in the amount of reducing agent used led to significantly smaller particles. With a 1.5-fold increase in reducing agent concentration Ag:Au-nanoparticles with nine different compositions and particle sizes of ~ 7 nm could be synthesized and functionalized with PVP. Interestingly there was almost no trend in particle size observable. TEM- and XRD-studies showed that the nanoparticles synthesized with a 1.5-fold amount of citrate/tannin were nanocrystalline and not twinned. Furthermore a distinct compression of the unit cell was observed, leading to a deviation from Vegard’s law and the emergence of microstrain in the crystal lattice of the nanoparticles. Cellbiological studies revealed a toxic effect of the nanoparticles towards HeLa-cells and hMSCs which was dependent on the total metal concentration and the silver content of the nanoparticles. However, the dependence on the silver ratio was not linear, as particles with a silver content of below 50 mol% showed only a slight toxic effect while Ag:Au-80:20-nanoparticles exerted higher toxicity than Ag:Au-90:10- and pure Ag-nanoparticles after an incubation time of 24 h (total metal concentration hMSCs: 50 µg mL-1, HeLa-cells: 50 µg mL-1 and 100 µg mL-1). In addition to Ag-Au-nanoparticles in the size range of ~10 nm, ultrasmall cluster-like Ag:Au-nanoparticles were synthesized via simultaneous reduction of Au3+ and Ag+ with sodium borohydride in the presence of thiol-ligands, i.e. 11-mercaptoundecanoic acid and dihydrolipoic acid. With 11-MUA as ligand, monodisperse, quasispherical nanoparticles were synthesized, that could be purified by precipitation and redispersion. The 11-MUA-Ag-Au-nanoparticles had a mean diameter of about 2 nm, independent of their Ag:Au-composition. The theoretical ratio of Ag and Au was confirmed by AAS. By 1H-NMR and ATR analyses it was revealed, that the ligand was bound to the particle surface with the thiol-group. 11-MUA-Ag-Au-particles with a composition between Ag:Au-10:90 and Ag:Au-60:40 showed notable autofluorescent properties with emission in the visible range. The color of the emission seemed to depend on the silver content of the nanoparticles and showed a red shift with increasing Ag:Au-ratio. Cellbiological studies revealed a rather low toxicity of the particles towards HeLa-cells with cell viabilities mostly above 50 % after incubation for 24 h with a total metal concentration of 50 µg mL-1. Only nanoparticles with a very high silver content exerted stronger toxic effects. Uptake studies with autofluorescent 11-MUA-Ag-Au-nanoparticles indicated, that the particles accumulated mainly on the cell membrane and presumably in endosomal vesicles.