To study the osteoinductive action of hyaluronic acid (HA), we examined the effects of applying an elastoviscous high-molecular HA preparation on bone wound healing after bone marrow ablation. The middiaphyses of cortical bones from rat femurs were perforated with a round bar, and excavated marrow cavities were filled immediately with high-molecular HA. Bone marrow ablation without HA was used to prepare controls. On post-ablation days 1, 2, 4, 7, and 14, animals were perfusion-fixed with an aldehyde mixture, and dissected femurs were examined by means of light, transmission-, and scanning-electron microscopy. In controls, the wounded marrow cavities were first filled with blood and fibrin clots (days 1 and 2), then with granulated tissues containing macrophages, neutrophils, and fibroblastic cells (day 4). New bone formation by differentiated osteoblasts was observed at 1 week post-ablation; at 2 weeks, the perforated cortical bones and marrow cavities were filled mostly with newly formed trabecular bone. In bones to which HA had been applied, new bone formation already had been induced by day 4 on both the peri- and endosteal surfaces of the existing cortical bones. At 1 week post-ablation, marrow cavities were completely filled with newly formed trabecular bones, in which active bone remodeling by osteoblasts and osteoclasts had occurred. Granulated tissues were replaced rapidly by normal marrow cells. These results suggest that high-molecular HA is capable of accelerating new bone formation through mesenchymal cell differentiation in bone wounds.