Although diamines and polyamines are widespread in biological material [ 1,2] little is known about their physiological role. Numerous studies have established the metabolic relationship of the putrescine (1,4-diaminobutane) and the polyamine spermidine. In various systems, putrescine serves as a precursor for spermidine, which is synthesized by the condensation of the diamine with S-adenosyl-L-methionine [ 1,2] . The biosynthesis of putrescine, on the other hand, is catalyzed by ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) which converts ornithine to putrescine [2]. Coliphages of the T-even series contain considerable amounts of polyamines [3-S] . According to Ames and Dubin [S] ,8 X 10” Escherichia coli B cells, contain 11.7 nmoles putrescine and 1.6 E.tmole spermidine. These cells give rise to 5 X IO’ 3 T4 phages which contain 12.5 fitmoles putrescine and 3.75 pmoles spermidine. It thus appears that phage-infected bacteria should synthesize polyamines at a fast rate, to supply the polycations required for the phages and to make up for the polyamines (up to 50% of the putrescine) which leak from the cells soon after infection [6-81. In the present paper we demonstrate the stimulation of ornithine decarboxylase activity after infecting E. coli with T2 phage. It will be shown that the activity is maximal 6 to 9 min post-infection.