The diffusion properties of fluorescent colloidal CdSe and CdSe/ZnS nanocrystals (QDs) with different hydrophilic coatings were characterized in complex fluids such as actin solutions using fluorescence correlation spectroscopy (FCS). The hydrodynamic radii of the QDs were determined both in organic solvents and water. Attention was given to the potential artifacts arising from the fluorescence properties of the QDs. With increasing excitation intensities, the apparent particle concentration and diffusion times are overestimated if using a simple diffusion model. This can be explained by a numerical simulation. The diffusion behavior of QDs in actin networks of different concentrations was determined to demonstrate the potential use of nanocrystals as probes in soft biological matter. The decreasing diffusion coefficient of the nanocrystals with increasing actin concentration results in an intrinsic polymer viscosity of 0.12+/-0.02 ml mg(-1), in accordance with literature values.