The effects of pyocin Rl on transport systems and syntheses of macromolecules in Pseudomonas aeruginosa strain P14 were studied under two conditions, 0.4 mM and 40 mM Magnesium ions. In the presence of 0.4 mM magnesium ions, the incorporation of [14C]leucine into protein and the uptake of [14C]leucine into the leucine pool were completely inhibited by pyocin. Furthermore, [14C]leucine which had been taken up into the leucine pool was depleted by pyocin. Comparing the concentration of amino acid within cells with that in the medium, it was concluded that the active transport in P14 cells was inhibited completely by pyocin. Such an effect of pyocin Rl on transport systems was not specific to leucine but common to many amino acids and uridine. On the other hand, under the condition of 40 mM magnesium ions, [14C]-leucine once accumulated was not depleted by pyocin, but uptake of [14C]leucine into the leucine pool stopped immediately while incorporation of [14C]leucine into protein continued at a considerably reduced rate; therefore the machinery for protein synthesis was not directly inhibited by pyocin. It is suggested that the loss from the amino acid pool in cells brings about the rapid cessation of protein synthesis under the condition of 0.4 mM magnesium.