mRNA degradation in Escherichia coli is mediated by a combination of exo- and endoribonucleases. We present evidence for a multiprotein complex which includes at least two enzymes that play important roles in mRNA degradation: the exoribonuclease polynucleotide phosphorylase (PNPase) and the endoribonuclease RNase E. An activity which impedes the processive activity of PNPase at stem-loop structures also appears to be associated with the complex. This complex is estimated to have a molecular mass of about 500 kDa and includes several additional polypeptides whose functions are unknown. The identification of a complex which includes several activities associated with mRNA degradation has implications for the mechanisms and co-ordinated control of mRNA degradation.