Summary Cultures of Pseudomonas aeruginosa in milk were ingested by three normal people. The organisms were recovered from the faeces if 106 or more were ingested. They were present in the greatest number in the first specimen passed, provided this was 12 hr or more after administration of the organisms. The numbers excreted were less than those ingested and excretion did not continue longer than 6 days. Colonisation was achieved on one occasion when ampicillin was also administered.