The fibrous network of those agarose gels typically used for electrophoresis is too fine to observe by light microscopy. In the present study, light microscopy, apparently for the first time, reveals the fibrous network of an agarose gel that has a micrometer-scale fibrous network. The agarose used is a derivatized, irradiated agarose previously shown by electron microscopy to have the following characteristic: When a 1.5% gel is cast at 30 degrees C, but not at 10 degrees C, fibrous bundles of agarose chains partition to form wider fibers that, in turn, form a network that has micrometer-sized and larger pores. When observed by phase-contrast light microscopy, gels cast at 30 degrees C have a clearly defined fibrous network not present for gels cast at 0 degrees C. The temperature-dependence of the network observed by light microscopy indicates that this network is the same as the micrometer-scale network observed by electron microscopy. For the network observed by light microscopy, the mean chord length varies inversely with the agarose concentration. The least concentrated gel observed had a concentration of 0.3%. Slight pressure on a 0.4% gel caused microfracture of the gel, thereby producing pores of a dimension between 5 and 50 mu m. These pores, freely permeable to latex spheres, will be useful for determining the hydrodynamic damping of intrapore, single-particle motion by gel fibers. (C) 1994 Academic Press, Inc.