While engaged in experimental researches on the introduction of insect flagellates, belonging to the genera Herpetomonas and Crithidia, into vertebrates, we have recalled to mind some former observations of ours made in 1909. When examining the blood of mice artificially infected with the spirochaetes of relapsing fever in the Quick Laboratory, Cambridge, we observed flagellates in the peripheral blood of the mice. These flagellates, which were scanty, were studied in fresh preparations by us and drawings of the organism were made with the camera lucida. Some of these drawings are now reproduced, together with others made by us subsequently. The flagellates were also observed by Professor Nuttall and his laboratory assistant, Mr B. G. Clarke, and we were unanimously agreed that the flagellates belonged to the genus Herpetomonas, or Leptomonas as some have since preferred to call it. The significance of the presence of herpetomonads in the blood of mice was very puzzling at the time, and pressure of other work prevented the subject being followed up. During 1911 and 1912, these flagellate organisms have been seen by all of us, but they were always evanescent and could not be found in fixed and stained preparations. The parasites were seen on different occasions in three mice altogether.