Fibroblasts were cultured on top of or at the bottom of hydrated collagen lattices. Shortly after initially interacting with the collagen lattices, fibroblasts appeared to attach to individual collagen fibrils and in many cases cell processes were found wrapped around clusters of collagen fibrils. Tension generated by cells during spreading resulted in proximal collagen fibrils becoming aligned distal in the plane of spreading and more densely packed. During subsequent culture, the collagen fibrils to the cells underwent a similar reorganization and the lattice thinned to one-tenth of its original thickness. The rate of thinning was similar regardless of whether the cells were originally above or at the bottom of the lattices. The presence of cells distributed throughout the lattice was unnecessary for lattice reorganization to occur. When the lattices were allowed to come off the underlying substratum, compaction of the collagen gels was observed, and the resulting matrix had the typical appearance of dermis as observed by both light and electron microscopy. Collagen fibrils associated with the cell surface often appeared to be under tension and, in regions of close fibril binding, there was a prominent reorganization of submembranous microfilaments. It is suggested that reorganization of the collagen lattice by fibroblasts may depend upon secreted cell factors as well as physical forces generated by the cells.