1. By means of phage typing of Bact. typhosum, the source of infection in a boy who drank water from a polluted stream was traced backwards via water, sewage effluent, crude sewage and faecal specimens to unsuspected typhoid carriers in a large institution.

2. A nurse in the institution contracted typhoid fever, caused by the same phage type (C) of Bact. typhosum as was found (a) in the stools of typhoid carriers in the wards in which she was on duty, (b) in the crude sewage and sewage effluent from the institution, (c) in the stream into which the sewage effluent discharged, and (d) in the stools of the boy who drank the stream water. The nurse's sister, who had had an attack of typhoid fever a few months earlier, was infected with a different type (E 1) of Bact. typhosum and was therefore not the source of her sister's infection.

3.The laboratory technique is described for the examination of (a) samples of water, sewage effluent and crude sewage, using different selective and enrichment media, (b) large numbers of faecal specimens for enteric organisms by a preliminary pooling of specimens in batches of six or seven, and further examination of the individual specimens comprising a positive batch, thus effecting a saving of time and economy of media.

We wish to thank the Medical Officers of Health, sanitary inspector and the medical and nursing staff of the institution for their enthusiastic co-operation in the investigation.