The Presumptive Coliform test is used as an indicator of possible faecal contamination of water supplies. In the attempts to provide a bacteriological standard for milk of good keeping quality which is safe for human consumption, the same test has been applied to milk. Thus, milks of high coliform content tend to be regarded by Department of Health authorities as "not up to standard" and not suitable for marketing. This view antagonises the farmer or producer who suffers financially if his milk is withheld from the market. His immediate reaction is to condemn the coliform test as valueless because it is not a true indicator of the standard of milk. The controversy arises because, for water samples, the presumptive coliform test is a good indicator of faecal contamination but has quite a different significance in milk. A positive test with milk may indeed indicate faecal contamination but there are many other possible explanations. Above all coliforms quickly die in water and their presence therefore shows recent faecal contamination; whereas in milk the organisms not only do not die but grow abundantly. Opinion differs greatly on the validity of the test, as can be seen from the fact that in England the test was abolished in 1949 whereas it is still retained in Scotland, In America it is retained for milk of "Certified" grade. The chief aim in milk production should be to provide the consumer with milk of good keeping quality and free from any pathogenic bacteria. How does the presumptive coliform test stand in this matter? In Scotland there are three main grades of milk namely, "Certified", Tuberculin Tested (T.T.) and Pasteurised milk, and / and each has its own coliform-test standard, "Certified" milk comes from tuberculin-tested herds, is produced under very clean conditions, and is bottled on the farm. It is regarded by many people as the "best" milk. T.T, milk comes from tuberculin-tested herds and may or may not be pasteurised. In Scotland 99.8% of herds are T.T. (Chalmers and Sampson, 1958). Pasteurised milk has undergone heat treatment to destroy any pathogenic bacteria which may be present. Undoubtedly it will be the safest milk, but many people dislike its 'flat' taste and are under the impression that it loses some important fraction of its nutritive value after heat treatment. With the above points in view, it was decided to examine milks of different grades, at different seasons, as purchased by the consumer i.e. milk delivered in bottles after it had passed through all stages of production. This was decided upon because most routine testing of milk is done before the milk reaches the consumer, usually on individual samples from farms or on bulk samples at creameries. It seemed of interest to discover whether the bacteriology of the milk would shed some light on the possible status of the test by the time that the milk had actually reached its destination. Presumptive coliform tests were carried out, and positive samples were further examined to see whether coliform organisms were indeed responsible for the positive tests. Samples were also examined by other tests and some interesting points arose. One of the most surprising was the high incidence of Staphylococcus aureus in raw milk, and the revealing fact that on bacteriological grounds "Certified" milk is far from being a "good" milk compared with Pasteurised milk. The following work is therefore directed mainly towards findings on the examination of milks by the presumptive coliform test. It also includes, however, a number of relevant observations on the significant bacterial flora of consumer milk
