Felix d'Herelle and the controversey on the Twort-d'Herelle phenomenon in the 1920's: ultrafilterable virus or lytic ferment This paper deals with a controversy on the Twort-d'Herelle phenomenon — also called bacteriophagy — which continued during the 1920's and 19.30's. This phenomenon is characterised by the marked lysis of bacteria by a so-called 'lytic principle'. The article is focused on one of the important persons in this controversy: Felix d'Herelle. The idea that the 'lytic principle' is a living ultrafilterable virus (F. d'Herelle) was opposed to that which describes it as a ferment (J. Bordet). In the beginning of the scientific dispute the criteria between living and dead were playing a preponderant role. Later on, the discussion moved to the question whether the 'lytic principle' is a product of the bacterium or that it is exogenous. Both positions were able to explain most of the experimental findings. The phenomenon attracted a lot of bacteriologists and pathologists because of its supposed potentialities with regard to immunity against bacterial infections