Based on a similarity of cultural and biochemical properties, WILSON and MILLES (6) compared the microorganisms of listeriosis with those of swine erysipelas including them in the genus Erysipelothrix with the species E. monocytogenes and E. rhusiopathiae. However, BARBER (1) and JULIANELLE (4), in a comparative study of both microorganisms, do not admit that possibility. Observing the morphological and citochemical characteristics of L. monocytogenes (N.º 7 973, 5 348, 5 105, 5 214, of the Seeliger collection) and E. ehusiopathiae cultures (N.º 1, 7, 11, 27, 37, of the WIX collection), both in phase "S", we were able to verify their comparative behavior when in contact with 2-3-5-triphenyltetrazolium chloride (TTC), "Synthetical Laboratories, Chicago, U.S.A". 1. To 1,0 ml of cultures of both microorganisms grown in plain broth plus 5% horse serum for 24 hours at 37ºC, 0.1 ml of sterilized 1% aqueous solution of TTC was added. The tubes were left at room temperature (± 25ºC) or in the incubator (37ºC). In the tubes with L. monocytogenes immediately appeared a red coloration (fig. 1) characteristic of formazan production, that became gradually more pronounced, from bottom to top (figs. 2, 3, 4, 5); at the and of 24 hours the color was intense the tube. In the tubes with E. rhusiopathiae (figs. 1, 2, 3, 4), only a faint pink coloration was seen after 2 hours; at the end of 24 hours, the intensity of color was not equal to that observed in tubes with L. monocytogenes. 2. Petri dishes with 15 ml of plain agar with 5% horse serum, containing 0.1 ml of 1% sterilized aqueous solution of TTC, were inoculated with L. monocytogenes and E. rhusiopathiae, and incubated for 24 hours at 37ºC. Colonies of L. monocytogenes that developed presented a red color, indicative of formazan, as previously described by GRAY et al. (3), while the E. rhusiopathiae colonies did not show any coloration. 3. Disks of filter paper, previously soaked with TTC and dried were placed over the growth of L. monocytogenes in Petri dishes with plain agar plus 5% horse serum form 24 hours. No change in the color of the colonies was observed. The same negative result was obtained when the porous clay cylinder technique for measuring antibiotic activity was employed, With E. rhusiopathiae also negative results were obtained with the disk technique. 4. Wet preparations of the cultures after 1 hour of contact with TTC in the same conditions of item 1, were observed at 900X magnification. L. monocytogenes presented normal shape and dimensions, but one to three intracellular polar, bipolar and or central, red-purple granules were noted. E. rhusiopathiae bacilli were morphologically normal…