The use of metronidazole, 1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole, in the treatment of infections due to Trichomonas vaginalis was first reported by Durel, Roiron, Siboulet & Borel (1960). Subsequently, it was shown to be effective in infections due to Entamoeba histolytica (Powell, MacLeod, Wilrnot & Elsdon-Dew, 1966), and Giardia lamblia (Schneider, 1961). An astute observation by Shinn (1962), who noticed that a patient with ulcerative gingivitis being treated with metronidazole for trichmonal vaginitis recovered from both conditions, stimulated further studies which confirmed the efficacy of this agent in ulcerative gingivitis (Shinn, Squires & McFadzean, 1965). Since the metabolism of the organisms causing the infections above described is primarily anaerobic, it was natural that investigators should examine the susceptibility of anaerobic bacteria to metronida.zole, both in vitro and in vivo, and a number of papers describing the results of these investigations have appeared in recent years (vide infra). It is pertinent now to review the current literature on the antibacterial activity of this agent and assess its role in the treatment of anaerobic infections.