How well can the diffusion coefficient D of a globular biomolecule be predicted from its molecular mass MW? In “Wanted: Scalable Tracers for Diffusion Measurements” [J Phys Chem B, submitted], I propose that diffusion measurements in heterogeneous systems can be improved by the use of scalable tracers, in which the size is varied alone at constant shape, surface properties, diffusion mechanism, deformability, and other properties affecting diffusion. Before trying to design a de novo series of scalable globular proteins, it is appropriate to examine how scalable the commonly used de antiquo globular proteins are [ibid., supporting information]. The widely-used compilation of experimental diffusion coefficients by Tyn and Gusek [Biotech Bioeng 35 (1990) 327] was examined. This set -- ranging from ribonuclease, 12640 Da, to tobacco mosaic virus, 50 MDa -- was plotted as D versus log MW. The obviously linear species were removed, and values of D and MW for the outliers were examined. The plot yields a cloud of values of D versus log MW. In this plot, rigorously scalable tracers are expected to give a single smooth curve of D versus log MW, and the extent of the cloud represents scatter due to nonscalablity in the other properties, and to experimental error. Values of D from hydrodynamic calculations from various laboratories are remarkably consistent with the cloud. The cloud prediction is certainly good enough for semi-quantitative estimates or for designing single-particle tracking experiments. For a diffusion-controlled reaction in dilute solution, the prediction is close enough that the standard analysis of propagation of errors can be used. But arbitrary cloud proteins are not adequate for, say, measuring the percolation threshold of cytoplasm. The incomplete examination of the question here indicates what would be required for a complete examination.