Mesh sizes in concentrated aqueous solutions of collagen and hyaluronic acid were measured using an electron spin resonance (ESR) based technique. The technique involved spin labeling poly-(acrylic acid) probe molecules of various sizes to serve as sensors which were mixed into the concentrated polyelectrolyte solution. When probe molecules were smaller than the mesh size of the polyelectrolyte solution, the local rotational correlation time (RCT) of the spin label was essentially unchanged from free solution. When the mesh size approached or was smaller than the probe size, the RCT was altered due to close-range interactions between the probe molecule and neighboring polymer chains. The smallest size probe that interacted with a polymer matrix was used as the basis for quantifying mesh sizes. There was reasonable,agreement between theoretical estimates of mesh sizes in collagen solutions and estimates from the experimental technique. Measured mesh sizes in hyaluronic acid solutions show good quantitative agreement with reported mesh size calculations from measured probe diffusion rates in similar matrices.