The rimI gene of Escherichia coli K12, which encodes an enzyme catalysing acetylation of the N-terminal alanine of ribosomal protein S18, has been cloned into a mini-F plasmid pRE432 and characterized at the molecular level. Similarly, the rimJ gene, which encodes another acetylating enzyme that is specific for ribosomal protein S5, has been cloned and characterized. From the nucleotide sequence data for the two genes the RimI enzyme was deduced to contain 161 amino acid residues with a calculated molecular weight (Mr) of 18232 and the RimJ enzyme contains 194 amino acid residues with a calculated Mr of 22687. The proteins produced from the two genes in maxi-cells were identified by electrophoresis on acrylamide gels and their operon structure was analysed by insertional mutagenesis with transposon gamma delta (Tn1000) and by measuring the size of their transcripts. Their structural homology was analysed by DNA hybridization and by calculation with computer programs. There is only a low level of overall homology between the two genes except for a 3' terminal region in which a significant degree of homology was noticed.