An investigation was done on the action in vitro of two pharmaceutical preparations containing Bi, De Nol and Pepto Bismol, on the fermentative capacity of intestinal bacteria. For the purpose, using an instrument and a technique previously described, fecal fermentation (FF) was measured in 21 stool samples, to which lactose was added to assure appreciable levels of fermentation. In 11 of the samples, FF was measured without and with addition of De Nol; and in the 10 remaining samples, without and with addition of Pepto Bismol. In 7 of the samples with which the action of De Nol was studied, this preparation significantly reduced FF from 10.9 not equal to 4.3 (mean not equal to s. d.) to 4.9 not equal to 5.0 ml of gas/24 h (p less than 0.02); on the contrary, in the 4 remaining samples, it increased FF, although not significantly, from 8.7 not equal to 4.8 to 16.7 not equal to 8.8 ml of gas/24 h (N.S.). Pepto Bismol significantly reduced FF, in all the 10 samples with which the action of that pharmaceutical - preparation was investigated, from 11.7 not equal to 6.1 to 3.0 not equal to 2.1 ml of gas/24 L (p less than 0.0001). A discussion was done on the possible mechanisms of the antifermentative action of Bi, as well as on the probable usefulness of De Nol and Pepto Bismol for the control of exaggerated intestinal fermentation and the flatulence it frequently causes.