We have previously shown that the presence of fibronectin (FN) and/or hyaluronic acid (HA) in a 3-dimensional type I collagen sponge enhances wound healing in vivo. In the present study the same material was used as a support for growth of fibroblasts in vitro. Using radiochemical techniques, scanning electron and light microscopy, the properties of fibroblasts cultured on the collagen sponge or on the sponge containing HA or FN have been compared with cultures grown on plastic dishes. Fibroblast replication and collagen synthesis were higher on plastic than on the collagen sponge. In the presence of HA or FN the entire thickness of the sponge was infiltrated by fibroblasts which rapidly replicated. The presence of HA or FN increased synthesis of collagen which was largely deposited around cells.