Molecular crowding in highly concentrated monoclonal antibody (mAb) solutions results in significant increases in viscosity, which complicates fill-finish steps and patient administration by subcutaneous injection. As viscosity measurements for optimization of the mAb formulation require significant amounts of material not always available in early development, fluorescence correlation spectroscopy (FCS) is evaluated as a potential ultralow volume technique for viscosity measurement of high concentration protein solutions assuming the Generalised Stokes Einstein relation (GSE) remains valid. Using like-charge fluorescent tracers of different sizes, FCS provided measurements of microviscosities which were compared to the macroviscosity. After parametrising the protein concentration dependence of the viscosity by the exponential coefficient (k) of a simple exponential model, FCS derived k-values of like-size tracer to the crowder followed the same ordering as the macroviscosity derived k-values with respect to solvent conditions. Furthermore, k and the diffusion-derived protein-protein interaction parameter, kD, are linked, and, attractive conditions for mAbs result in a stronger concentration dependence of the viscosity. For tracers and crowders of like-size, a key result is negative deviations from the GSE relation are observed in presence of strong attractive interactions between crowder molecules. These data demonstrate that FCS has application to the screening of high concentration mAb solutions for formulation selection.