Earlier studies had shown that a large portion of bacterial messenger RNA carries 3'-terminal polyadenylate sequences, albeit of somewhat shorter length than those associated with eukaryotic mRNA. In this paper, we show for the first time that a specific prokaryotic mRNA is polyadenylated. Three independent lines of evidence demonstrate that a 3'-terminal polyadenylate sequence 10 to 15 nucleotides in length is associated with about 40% of the mRNA of the outer membrane lipoprotein of Escherichia coli: 40% of lipoprotein mRNA binds to oligodeoxythymidylate-substituted cellulose at high ionic strength and is eluted by water; treatment of lipoprotein mRNA with oligodeoxythymidylate and ribonuclease H destroys its ability to bind to oligodeoxythymidylate-cellulose; and in the presence of oligodeoxythymidylate, lipoprotein mRNA can serve as template for the synthesis of DNA complementary to lipoprotein mRNA by reverse transcriptase. In view of the fact that the lpp gene and its downstream-flanking region contain no continuous deoxyadenylate sequences longer than five nucleotides, the polyadenylate moiety must be added post-transcriptionally. It was possible to demonstrate the synthesis of polyadenylated lipoprotein mRNA in toluene-permeabilized cells of E. coli, opening the way for the study of its biosynthesis.