1. The presence of polyamines in the growth medium of Escherichia coli can modulate the activity of the RNA-processing enzyme, ribonucleoprotein ribonuclease P (RNase P), by altering the expression of the rnpA and rnpB genes, which encode its C5 protein and M1 RNA subunits, respectively. 2. Following growth in the presence of 1 mM spermidine the levels of C5 protein mRNA and catalytic M1 RNA were significantly elevated in the wild type E. coli K-12 strain MG1655. 3. The rnpA mRNA, together with the ribosomal protein L34 (rpmH) mRNA, was found to constitute a dicistronic rpmH-rnpA message whose half-life did not change upon Escherichia coli growth in the presence of spermidine. 4. This suggests that the spermidine effect is on the transcriptional level. 5. Increased expression of the rnpA and rnpB genes was reflected in the activity of RNase P, which almost doubled. 6. These results identify yet another component of the protein synthetic machinery which is specifically affected by polyamines.