Abstract
An activity in fractionated extracts from E. coli which reduces the infectivity of the replicative form of phage fd DNA is correlated with the in vivo restriction phenomenon by its presence only in fractions from restricting strains of bacteria and its specificity for nonmodified DNA. The inactivation which requires S-adenosylmethionine, ATP, Mg++ and the products of at least 2 gene functions; its seems to be accompanied by double-strand cleavage of the DNA.