Five antimicrobial ammonium cations were intercalated into two clay minerals (saponite and montmorillonite) by using ion-exchange reaction. About 90% of the cation exchange capacities of the clay minerals were replaced by ammonium ions with a few exceptions. The resultant complexes exhibited antimicrobial activity against both Staphylococcus aureus and Escherichia coli. After heating to 250-degrees-C, the antimicrobial activity was kept for the complexes with aromatic ammonium ions but not for those with aliphatic ammonium ions. The complexes were treated with deionized water and a 3% aqueous solution of sulphuric acid to examine the elution of the ammonium ions. The elution finished within 1 day, with retention of 80% of the aromatic ammonium ions. Aliphatic ammonium ions remained in smaller amounts.