editor hesitates to accept the use of the term "antibiotic" for anti­ microbial agents of non microbiological origins which is sometimes encountered, but neither does he subscribe to the view that antibiotics are in some fundamental manner different from chemotherapeutic substances of other origins. Modes and mechanisms of action of chemotherapeutic compounds are not systematic functions of their origins nor of the taxonomical position of the target organisms. Consequently, in the selection of topics for Antibiotics III (published in 1975), synthetic drugs and natural products of higher plants (alkaloids) were represented, along with antibiotics in the strict sense of the definition. We now present Antibiotics V, for whose assembly the same selection criteria were applied as for Antibiotics Ill. The aggregate length of the contributions rendered it impractical to place the entire text between the covers of one book.