Mn(3)O(4) nanoparticles with diameter ca. 10nm were synthesized by the forced hydrolysis of Mn(II) acetate at 80 degrees C. The X-ray diffraction (XRD), Fourier transform infra red (FT-IR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques were employed to study structural features and chemical composition of the nanoparticles. The unique oxidative activity of the Mn(3)O(4) nanoparticles was demonstrated in the polymerization and dye degradation reactions. On adding Mn(3)O(4) suspension to an acidic solution of aniline, yielded immediately green sediment of polyaniline (PANI). The organic dyes, viz., methylene blue (MB) and procion red (PR) were found to be completely decolorized from their aqueous solution on treating the dyes with Mn(3)O(4) suspension in acidic media. The Mn(3)O(4) nanoparticles also showed a clear antibacterial activity against the Vibrio cholerae, Shigella sp., Salmonella sp., and Escherichia coli bacteria that cause cholera, dysentery, typhoid, and diarrhea diseases, respectively.