The selective and suppressive mechanism of Rappaport's medium as related to the growth of different species of Enterobacteriaceae was evaluated. With the hyperionic medium, which includes 3.6 per cent magnesium chloride, growth of Salmonella typhimurium was suppressed, this specie being inhibited even with a 3.1 per cent concentration of the same compound in the medium. Suppression of growth for the other species was fownd at 1.8 per cent magnesium chloride for the Shigellaei, 2,2% for Escherichia coli, 3,1% for Enterobacter sp., and only when reached 3,6% for Proteus mirabilis. Malachite green, at 0.012 per cent concentration, as in the original medium, was inhibitory for most tested species, with the exclusion of Entercbacter and S. typhimurium. By substituting acid sodium phosphate for the original potassium phosphate, a general decrease of the suppresive characteristic of the medium occurred. The four Shigella species were most sensitive to the magnesium chloride hypertonic solutions, among the enteric organisms tested, and similarly they did not grow in two other hypertonic media with calcium or magnesium thiosulfate, instead.