As is evident from the above summary of the recent literature, plus many other papers not cited here, there is an extensive literature indicating the physiological significance of these amines. The most important studies can be summarized as follows. (a) Polyamines and their biosynthetic enzymes are ubiquitous. (b) Microbiological mutants have been described in which there is a definite requirement of polyamines for growth. (c) The concentration of polyamines and their biosynthesis enzymes increase when the growth rate increases. These increases usually precede or are simultaneous with increases in RNA, DNA, and protein levels. (d) Ornithine decarboxylase has a remarkably fast turnover rate in animal cells, and the level of this enzyme rapidly changes after a variety of growth stimuli. (e) Polyamines have a high affinity for nucleic acids and stabilize their secondary structure. They are found associated with DNA in bacteriophages and have a variety of stimulatory effects on DNA and RNA biosynthesis in vitro. (f) Polyamines stimulate protein synthesis in vivo and in vitro. (g) Polyamines protect spheroplasts and halophilic organisms for lysis, indicating their ability to stabilize membranes. Despite these observations, no specific mechanism has been firmly established for the action of the polyamines in vivo. It is clear that these compounds are physiologically important, however, and further work is necessary to establish the mechanism of their action.