Things move quickly in the domain of antibiotics, and the book of today is easily obsolete tomorrow. One welcomes, therefore, a new edition of so important a treatise as Dr. Waksman's. In this revision the number of pages has grown to 415 from 350 in the old edition; the number of references is now 1,053 instead of 1,016. Nowhere is progress more evident than in the chapter on the "chemical nature of antibiotic substances," in which appear many precise formulas not yet worked out at the time of the first edition. The Commonwealth Fund has done a real service to medicine in producing a scholarly and authoritative compendium of knowledge in such an important field.