PURPOSE: An antimicrobial agent useful as a medical germicide, a disinfectant for kitchen, or for dairy farming, comprising a specific mono-N-tetradecylpolyalkylenepolyamine or its acid addition salt as an active ingredient.

CONSTITUTION: The titled agent comprising a mono-N-tetradecylpolyalkylene polyamine obtained by alkylating one animo group of a compound shown by the formula (n is 0W3; m is 2 or 3) with C14H29- group or its inorganic acid addition salt as an active ingredient. When the titled active ingredient itself is water-soluble, it is usable individually as an aqueous solution, and when it is water-insoluble, a solvent and an activator such as nonionic surface active agent are used as solubilizing agents to adjust the desired composition for use. The active ingredient is obtained by reacting tetradecyl chloride with a polyalkylenepolyamine (e.g., diethylenetriamine) shown by the formula in the presence of an alkali substance at 140W180°C.