Everyone knows that acquired microbic resistance is the Achilles heel of chemotherapy. All living things can acquire some resistance to noxious agents-man to morphine, for instance. Bacteria are no exception: they can become more resistant in some degree even to ordinary germicides. But nothing in the whole of nature equals the resistance which can develop to antibiotics; it seems here as if the sharper the weapon the more completely it can be blunted. A 10-fold change can deprive an antibacterial drug of much of its usefulness, but some increases are of the order of 1000-fold or even more. Since this change is a prolific cause of treatment failure it is worth while to inquire how it can be avoided.