The antibacterial properties of silver modified montmorillonites from Pellegrini Lake, Argentina were tested in growth inhibition of Escherichia coli bacteria. Montmorillonite was first submitted to different treatments: (a) calcination at 550C for 3 h and (b) grinding during 300s. After that, the samples were loaded with silver by ion exchange. Structural characterization was performed by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), and BET specific surface area measurements. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTM) showed that metallic silver nanoparticles precipitates over the clay surface after silver modification. Nevertheless, the displacement of the (0 0 1) reflection observed by XRD in the calcined sample, and the diminution in Na+ content evaluated by energy dispersive X-ray spectroscopy (EDXS), indicate that Ag ions were interchanged in the structure of the clays. Both samples showed good antibacterial activity against E. coli, measured by the disk susceptibility and the minimum inhibitory concentration (MIC) tests. The ground montmorillonite required a lower MIC than the thermally treated, although the last one presented a bigger inhibition zone in the disk method. The results shows that the antibacterial activity is generated by the Ag+ present in the clay, as confirmed by X-ray photoelectronic spectroscopy (XPS); however the overall antibacterial proper-ties are affected by the availability of the ionic silver to be in contact with the bacteria. (c) 2007 Elsevier B.V. All rights reserved.