The swelling of a dextran gel, Sephadex G-75, was observed in an aqueous environment at room temperature by a noninvasive technique that uses light microscopy coupled to an image analysis system via a video camera. The rate of swelling was found to follow the Tanaka and Fillmore theory, from which the overall gel diffusion coefficient was estimated as 6.3 x 10(-7) cm2/s. In addition to giving a quantitative measure of gel swelling that could be useful in the mechanical design of liquid chromatography columns, this approach provides data on wet particle size and particle size range, which is needed for the modeling of diffusional and mass transfer effects in size-exclusion chromatography. In this context, key observations are that the gel particles are nearly spherical with an elliptical shape factor of 0.98 (perfect sphere = 1) and that there is little difference between sizes of particles obtained in water, 50 mM Tris-glycine buffer (pH 10.2), and buffer containing 1 mg/mL protein. The diameter of the dry material ranged from 20 to 100 microns, while the hydrated particles had diameters of 40-350 microns. The rate of swelling is rapid, with 50% swelling occurring in about 10 s and swelling to 99% of the final wet particle size being obtained in less than 90 s.