Provided are a microorganism-controlling agent that exerts excellent microorganism-controlling actions, in particular, an antibacterial action, an algicidal action and an action of preventing biofilm formation and yet shows suppressed metal elution, a miticidal composition that exerts an excellent action of suppressing the infestation and growth of mites, a mite-controlling material and a mite-controlling method. The microorganism-controlling agent according to the present invention contains platinum particles and particles of a metal other than platinum. In the microorganism-controlling agent, the platinum particles and the metal particles may be carried on a support. The microorganism-controlling agent exerts an excellent antibacterial action and shows suppressed metal elution. The miticidal composition according to the present invention contains platinum particles and particles of a metal other than platinum. The miticidal composition exerts an excellent action of suppressing the infestation and growth of mites.
