In recent years some attention has been focused on the low temperature storage of various protozoa. The purpose of this work was to confirm that Trichomonas vaginalis can be preserved for several months at a temperature of -196°C. in liquid nitrogen. Weinman and McAllister (1947), who successfully preserved a number of pathogenic protozoa, failed to preserve T. vaginalis, and only a proportion of their experiments with Trichomonas hominis were successful. No preservative was included in their cultures and the temperature was lowered only to -15°C. McEntegart (1954) preserved a number of species of Trichomonas at -79°C. in the presence of glycerol in a solid carbon dioxide/alcohol mixture; he obtained some viable T. vaginalis organisms after 4 months' storage, but there was evidence of slow deterioration. He suggested that a temperature of -190°C. would give better results. Lovelock and Bishop (1959) used dimethyl sulphoxide in place of glycerol as a preservative for the storage ofhuman and bovine red cells and of spermatozoa. This agent was suggested as a preservative preferable to glycerol because of its greater capacity for permeating cells in aqueous electrolytes. A slow permeation of glycerol into T. vaginalis would explain the comparative lack of success with this protozoon up to that time. Walker and Ashwood-Smith (1961) considered that dimethyl sulphoxide was less toxic than glycerol in the storage of trypanosomes. Diamond (1961) applied the use of dimethyl sulphoxide to the preservation of Entamoeba histolytica and successfully stored these organisms for 91 days in liquid nitrogen. Diamond, Bartgis, and Reardon (1965) applied a similar technique to the storage of T. vaginalis, 5 per cent. dimethyl sulphoxide being added as a preservative. Viable organisms were obtained after 2 years' storage in liquid nitrogen vapour at -170°C. The viability of the trichomonads was demonstrated by the fact that 29 of 30 mice died after intraperitoneal inoculation of a 48-hr-old subculture from the stored flagellates.