It has been shown previously that many aliphatic polyamines and metal cations decrease the infectivity of bacteriophage phi X174 when lipopolysaccharide from Escherichia coli C is present. Cations of higher charge are more effective. In the present paper it is shown that certain of the metal cations and cyclic polyamines diminish phage infectivity without lipopolysaccharide. The relation of cation concentration to loss of infectivity is different for the two types of reaction. In the absence of lipopolysaccharide the inhibition increases with the charge of the cation, but that by cyclic polyamines depends also on the hydrocarbon chains and their conformation. Some characteristics of the reactions are discussed.

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