In a case of suspected typhoid fever the examination of serum, blood and excreta would be necessary to determine whether the case were really typhoid or not. If found to be typhoid the laboratory would be called upon to try to discover the source of the infection. The same general methods hold good in other epidemic diseases where the laboratory is capable of making the diagnosis, to see whether any danger lurks in "contacts" or "carriers" and to find the source of the infection where possible. Very frequently material from wounds is sent in by the hospital surgeons to see whether wounds are infected. The soil of Flanders has been liberally manured for hundreds of years, and in every cubic yard of this manured soil are millions of the germs which cause gas gangrene and tetanus (lock-jaw) when introduced beneath the skin. If a wound is infected with gas gangrene or other dangerous organisms, the knowledge that they are present may materially modify the treatment used by the surgeon, and the laboratory is of value to him sometimes in determining that point. The usual routine work of a hospital clinical laboratory was also carried on by us for the casualty clearing stations in the area, and all kinds of work from the making of a vaccine for the treatment of bronchitis in a British General to the inoculation of a civilian child with anti-meningitis serum came within our scope. The hygiene work of the laboratory is also of a varied character: It consists of the examination of water supplies, milk and foods; the detection of poisons in water, and, occasionally, in human beings; the evolving of methods to purify effluents discharged into streams; work on poison gases and methods used to combat these, and many other things. In each division there are some sixty water carts, each of which holds about 110 gallons. We attempted to get samples from all of these in turn, to see whether the water had been disinfected. As all the sources of water supply in Flanders, with few exceptions, contain large numbers of bacteria, and as a properly chlorinated water contains very few bacteria, it is easy to tell from a couple of simple tests whether or not the water in the carts has been chlorinated. As we sometimes had eight divisions in our area at one time, this water control meant a good deal of work. The water carts were usually to be found at the headquarters of the unit to which they belonged, and we quickly discovered that the way to get the largest number of water samples in the shortest time was to travel by the map up and down the twisting narrow roads which intersected each other as though following the trails of the original inhabitants.