Electron microscopy of negatively stained samples shows that spermine and spermidine induce the polymerization of G-actin into filaments and paracrystalline bundles. In the presence of low concentrations of polyamines (0.02 mM spermine or 0.2 mM spermidine), filaments resembling salt-induced F-actin were observed, but as the concentration of polyamine was increased, relatively unordered bundles appeared. At about 0.2 mM spermine or 2.5 mM spermidine, the width of the bundles increased and they appeared more ordered with paracrystalline regions. This change was correlated with the gelation of the actin-polyamine mixture. At higher concentrations of polyamines (greater than 5 mM spermine or greater than 8 mM spermidine), the bundles had a similar ordered structure but, instead of gelation, there was an immediate precipitation of actin bundles. Spermine and spermidine also promote bundle formation from salt-induced F-actin. Two major paracrystalline forms were observed. Type I resembles the Hanson-type paracrystals induced by magnesium. Type II, characterized by an axial striation of 5.9 nm appears to be unique to polyamine-induced actin bundles.