The extracellular matrix in cultures of arterial smooth muscle cells has been examined by ultrastructural histochemistry using each of the following cationic dyes: ruthenium red, Alcian blue, acridine orange, and safranin O. All dyes exhibited an affinity for a structural component that was either preserved as a granule with ruthenium red or Alcian blue, or as an extended filament or bottlebrush structure with acridine orange or safranin O. Both granules and filaments were removed when the cultures were pretreated with chondroitinase ABC, an enzyme that degrades the glycosaminoglycan moiety of some proteoglycans. These structural components of the extracellular matrix were not observed when cultures were prepared in the absence of the cationic dyes. Labeling experiments (35S-sulfate) revealed that approximately 40% of the total labeled proteoglycans were lost during routine processing for electron microscopy (i.e., fixation through dehydration). Inclusion of any one of the cationic dyes during fixation reduced the losses to less than 1%. The extended filamentous structure preserved by safranin O and acridine orange resembled the structure of purified proteoglycans prepared from the same cultures and spread on cytochrome c monolayer films. These observations suggest that proteoglycans exist as extended bottlebrush structures within the extracellular matrix, and support the interpretation that the granular deposits observed in the ruthenium red and Alcian blue preparations most likely represent individual proteoglycan monomers that have undergone molecular collapse during processing. In addition, the dyes also exhibited an affinity for chords of fine fibrils that contained small granules and/or filaments. Both the fibrillar material and the associated granular and filamentous structures enmeshed in the fibrils resisted digestion with chondroitinase ABC.