The distribution of viscosities in living cells is heterogeneous because of the different sizes and natures of macromolecular components. When thinking about protein folding/function processes in such an environment, the relevant (micro)viscosity at the micrometer length scale is necessarily distinguished from the bulk (macro)viscosity. The concentration dependencies of microviscosities are determined by a number of factors, such as electrostatic interactions, van der Waals forces, and excluded volume effects. To explore such factors, the rotational diffusion time of myoglobin in the presence of varying concentrations of macromolecules that differ in molecular weight (dextran 6000, 10 000, and 70 000), shape (dextran versus Ficoll), size, and surface charge is measured with time-resolved linear dichroism spectroscopy. The results of these studies offer simple empirically determined linear and exponential functions useful for predicting microviscosities as a function of concentration for these macromolecular crowders that are typically used to study crowding effects on protein folding. To understand how relevant these microviscosity measurements are to intracellular environments, the TRLD results are discussed in the context of studies that measure viscosity in cells.