During my work in Yucatán (tropical Mexico) I observed that we nearly always found by the current analysis of urine, a well pronounced indican reaction, certainly in more than 80% of the cases examined; and frequently the blue colour obtained by the Obermeyer or Jaffé reaction was so strong that one would have been inclined, if in Europe, to postulate the presence of intestinal obstruction or some serious intestinal disorder. Experience soon taught us however that the reaction had not the same significance in Yucatán and probably in other tropical countries as it possesses elsewhere. With the object of studying the underlying causes I suggested to one of my assistants an investigation of the indole-producing power of B. coli isolated from the faeces of different individuals. The results which have been published in a dissertation (Hernandez, 1908) were naturally very incomplete but the work led me to realise the importance of a quantitative indole-determination, and, as I did not find described in the literature any method suitable for our purpose, I proceeded to work out a method for myself. I used the Salkowski test, with nitrite of potassium, sulphuric acid, and extraction with amyl-alcohol, preparing a series of tubes containing determined quantities of indole in progressive concentrations. The indole I distilled from bacterial cultures. The scale kept well, having been preserved for three years in the dark and only brought into the light when in use.