As several other surfactants do, the detergent Metaupon acts on the multiplication of bacteriophages. We investigated the influence of Metaupon on the phages 0 105 and 1, the cyanophage LPP-1, and the RNA-phages f 2 , M 12, and QB by means of the agar diffusion test, pour plate test, adsorption test, and one-step growth test. The action of Metaupon on the free phages was also tested. Metaupon inhibits the formation of plaques by the phages with exception of 3,. With the phages f 2 and M 12 the substance increases the amount of plaques depending on concentration. The main mode of action of Metaupon was found to be the inhibition of the adsorption of the phages to the host cells. Only in the case of CD 105 free phages were inactivated.Es ist bekannt, daB Detergentien auf Bakteriophagen wirken konnen. Als Wirkorte solcher Verbindungen wurden die freien Phagen, die Adsorption der Phagen an die