The functions of acetylpolyamines were examined with respect to stimulation of protein synthesis and cell growth. Unlike polyamines, acetylpolyamines could not lower the optimal Mg²⁺ concentration of protein synthesis, and the degree of stimulation of protein synthesis by acetylpolyamines was small. The addition of N¹-acetylspermine did not stimulate cell growth of a polyamine- requiring mutant of Escherichia coli MA261, although acetylspermine was accumulated in the cells. Acetylspermine did not interfere with polyamine stimulation of protein synthesis and cell growth of E. coli MA261. The binding of acetylpolyamines to RNA was very weak, and the binding of polyamines to RNA was not disturbed significantly by the presence of acetylpolyamines. When the growth of E. coli MA261 was stimulated by addition of polyamines, significant amounts of acetylpoly-amines were also formed in the cells. These results suggest that acetylation of polyamines, together with polyamine excretion, may regulate the intracellular level of the parent polyamines when excess amounts of polyamines accumulate intracellularly.