The occurrence and distribution of intermediate-sized filaments (100 A filaments) in an established epitheloid cell line RL 19 originally derived from rat liver was described by use of ultrathin sections tangentially and crossly to the cell substrate. The epithel-like cells showed in the cytoplasm a highly developed system of intermediate-sized filaments. The intermediate-sized filaments are either individually traversing in different directions or are grouped to loosely arranged bundles randomly running wavy-like through the whole cytoplasm. The bundles of highly variable thickness (20 to 40 filaments) are branched or are uniting and assembled to a three-dimensional network. The nucleus and Golgi-system are surrounding by a large ring of densely packed intermediate-sized filaments. Occasionally parallel running single filaments are associated to electron dense aggregates.