Young dogs (2 to 4 mos. old) were placed in warm and humid rooms (90° F. temperature, 85 to 90% humidity) for 3 days. They were given 100 cc. of cows'milk every 8 a. m., 12 noon, and 4 p. m. Half of the animals were given sanitary tap water and half were given polluted water (Chicago River water). No diarrhea developed in any of these 12 animals. Two hours before killing them in an ether chamber, they were given 100 cc. of their respective drinking water samples plus one agar slant of B. prodigiosus. The contents of various levels of the gastro-intestinal tract were diluted in broth and immediately seeded on agar plates. The relative distribution of B. prodigiosus was determined in relation to the concentration of the ingested suspension. Six puppies of about the same size were given the tap water and 6 the polluted water in ordinary temperature rooms. Chart I shows the results. There was an interference with the intestinal bactericidal power in the warm room; this was most marked when polluted water was ...