Metronidazole ("Flagyl", May and Baker), which has been inwide use for the treatment of trichomonal vaginitis for 6 years has also been shown to be highly effective in the treatment of Vincent's gingivitis (Shinn, 1962; Davies, McFadzean, and Squires, 1964; Shinn, Squires, and McFadzean, 1965). While it has not been possible to determine the minimum inhibitory concentration (MIC) of the compound against the spirochaetes thought to be involved in the aetiology of Vincent's disease, it has been shown to be active against the cultivable Reiter treponeme against which the MIC is 0-02 jig./ml. (Davies and others, 1964) compared to benzyl penicillin 0 005 ,ug./ml. These observations suggested that it might be effective against Treponema pallidum, although ScottGray and Murrell (1961) found no effect in a patient with primary syphilis who was given the drug for 48 hours (9 tablets, 1 * 8 g.). The present paper describes further observations made on patients with syphilis.