Polyamines, when added to cell-free protein-synthesizing systems, have been shown either to reduce mistranslation or to increase it depending upon the composition of the reaction mixture. To study this question under conditions as natural as possible we investigated the effects of polyamines on the fidelity of protein synthesis in intact Escherichia coli bacterial cells, using strains which were auxotrophic for polyamine synthesis. Error was measured in two ways: the incorporation of [3H]histidine into coat protein of bacteriophage MS2, the gene of which does not code for histidine, and the synthesis of a basic variant of MS2 coat protein in which a lysine erroneously replaces an asparagine, causing a change in isoelectric point. We found that when cell cultures were supplemented with polyamines there was no effect on the first type of error and the second type decreased twofold. The measured errors occurred at the level of translation because their frequency increased in the presence of streptomycin and decreased in cells bearing a streptomycin-resistance mutation known to lower the occurrence of translational misreading. The average erroneous incorporation per mol coat protein in the presence of polyamines was 1.43 +/- 0.59 mmol histidine and 25-34 mmol lysine/asparagine substitution. The reason for the different effect of polyamines on the two types of error is not known but could be related to the difference between their corresponding frequencies or to codon-specific effects.