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In recent years much work has been done on the bacterial content of milk, chiefly with a view to demonstrating the principal sources of contamination and at the same time collecting data to aid in the fixing of a bacterial standard for “clean” milk. On looking through the various publications one is particularly impressed by the lack of uniformity in the length of the period of incubation of gelatine and agar plates. Some observers adopt a period of only a few days while others prefer to incubate for 8–10 days before counting their plates. It does not yet seem to be generally recognised, in Britain at all events, that, if the highest figures possible are to be obtained, the incubation period must be longer than three days.