AbstractThe compartmentalization of eukaryotic cells by internal membranes and the subcellular localization of endogenous macromolecules by specific binding mechanisms are familiar concepts. In this report we present evidence that the cytoplasmic ground substance, which surrounds and contains the membranebound compartments, may also be compartmentalized by local differentiations of its submicroscopic structure that sort subcellular particles on the basis of size. The subcellular distribution of size‐fractionated, fluorescent tracer particles was studied in living cells by ratio imaging and fluorescence recovery after photobleaching (FRAP). Large and small particles showed different distributions within the cytoplasmic volume, suggesting that the large particles were relatively excluded from some domains. While the structural basis for this phenomenon is not yet understood in detail, ratio imaging of large and small particles can be used as an empirical tool to identify cytoplasmic compartments for further study. The cytoplasmic diffusion coefficient (Dcyto) and % mobile fraction of the large particles showed considerable spatial variation over the projected area of the cell, while Dcytoand % mobile fraction of the small particles did not. A model is presented to account for this difference. Based on this model, a method is proposed by which FRAP can be used to detect sol‐gel transitions in the cytoplasmic ground substance of living ce