A new fluorescent dye for the identification of trichomonas vaginalis is described. Specimens under investigation were mixed with 4',6-diamidino-2-phenylindol (Dapi) and examined using white light and UV-light. The combined use of the two techniques allows to recognize the mobile non fluorescing and the dead intensive yellow fluorescing trichomonads. Devitalized epithelial cells, bacteria and leucocytes in contrast to living trichomonas are stained yellow in a few minutes. This is of diagnostic help.