The precise functions of polyamines in mammalian cells have not yet been established, but the strong association between the rate of cell growth and proliferation and the rates of putrescine and spermidine synthesis suggests that they have an important regulatory role [ 11. The short half-lives of some enzymes required for polyamine synthesis, in particular the first enzyme of the pathway ornithine decarboxylase, and the consequent very rapid and extensive variation in its activity, reinforce this suggestion. Specific inhibitors of the production of polyamines would be of great value in further elucidating their functions, and one drug which has been widely used in this respect is the putrescine analogue 1,3-diaminopropane [2-171. Diaminopropane does not directly affect the activity of isolated ornithine decarboxylase, but greatly reduces the activity of the enzyme in the intact animal or cell [2,8,10,11]. As the enzyme synthesis can also be inhibited, apparently by a feedback repression mechanism, by low concentrations of its natural product putrescine [4,18,19], it seemed likely that diaminopropane was acting by a similar mechanism, although substantially higher concentrations of the analogue were required. However, it has been inferred from studies with virus-infected baby hamster kidney cells that the effects of diaminopropane may be due at least in part of a direct effect on cellular protein synthesis [20]. The dramatic effects on ornithine decarboxylase may thus be a function of its unusually rapid turnover rate rather than, or in addition to, any specific inhibition of its synthesis by a feedback repression mechanism. Here we report that diaminopropane, at concentrations within the range normally used, does indeed have a direct inhibitory effect on the rate of protein synthesis by a cell-free system from rabbit reticulocytes. The effect of diaminopropane is not due to any antagonistic effect against naturally occurring polyamines, but is simply a cation effect consequent upon the high levels used. In the absence of normal divalent cations, diaminopropane will itself support protein synthesis in this system. These results lend strong support to the doubts about the specificity of diaminopropane as an inhibitor of polyamine synthesis raised in [20].