Fracture healing, which involves a cascade of biological tissue responses, may be affected by various biochemical substances. One of these substances is tumor necrosis factor alpha (TNF). Studies were made on the effects of TNF on healing of fractured ribs of rats. Fracture healing was inhibited by daily administration of recombinant human TNF (400 micrograms/kg body weight per day, intraperitoneally) after fracture. The rate of union on day 20 was significantly lower in the TNF-treated group (4/18, 22.2%) than in the control group (14/18, 77.8%) (p less than 0.001 by Chi-square test). Histological examination showed that TNF inhibited cartilagenous callus formation. On day 10, cartilage was seen in the gap zone and under the periosteum in the control group, but no cartilage formation was observed in the gap zone in 9 of 12 specimens from the TNF-treated group. On day 20, the fracture ends were united by newly formed bone in the control group, but mature fibrous tissue was seen in the gap zone, and bony or cartilagenous union was not achieved in the TNF-treated group. These results show that TNF inhibits cartilage formation in the early phase of bone induction in fracture healing and suggest that this effect of TNF is due to its inhibition of differentiation of mesenchymal cells into chondroblasts.