Local treatment with phenol is often used after intralesional excision of chondroblastomas and giant-cell tumours which involve bone near joints, and has been shown to reduce the rate of recurrence. The ideal concentration of phenol is uncertain, but may be important because of the high rate of absorption and toxicity. We have studied the effectiveness of different concentrations on standard sarcoma cell lines. Our results suggest that a 5% solution of phenol is effective against dispersed single cells, and that higher concentrations give no significant advantage, but create problems due to lack of homogeneous mixing, temperature and safety.