Voltaire allegedly once defined a physician as "one who pours drugs about which he knows little into a body about which he knows less." Unfortunately, this somewhat cynical definition still has a large measure of truth to it in that too often reactions that are neither anticipated nor desired make their appearance. It is the purpose of this article to consider the mechanisms by which cutaneous side-reactions are produced. It is our impression that, as a rule, when a patient takes a drug and develops a reaction, cutaneous or otherwise, the glib assertion is made that the patient is allergic to the drug without any proof being brought forward to support the correctness of this designation. While there is no doubt that many reactions are based on allergic mechanisms, possibly oftener they are not; and we wish to emphasize the variety and pathogenetic diversity of the means by which cutaneous