WR 2721 (ethiofos) protects against the toxic effects of the heavy metal compound cisplatin. which is used in the treatment of solid tumours. In a Phase I protocol designed to determine the maximum dose of WR 2721 which could be tolerated when administered in combination with cisplatin and radiation therapy to patients with cervical carcinoma. 11 patients were evaluated by audiologic testing before and after cisplatin WR 2721 administration in an attempt to identify the degree of ototoxicity. Forty-five per cent were noted to have significant hearing threshold changes. predominantly in the high frequencies. There were no significant changes in the speech frequencies in this series. This contrasts with the greater degrees of ototoxicity observed in controls treated in the same way who received cisplatin without WR 2721 protection.