The designation “activated macrophage” was originally introduced to describe the state of macrophages that have an enhanced ability to phagocytize microorganisms and exert antimicrobial activity. This effect was found to be dependent upon particular infections and involves the participation of lymphocytes (1, 2). “Activation” has a specific immunologic basis, but its expression is nonspecific. Mackaness has written lucidly about activated macrophages and has referred readers back to the concepts of Lurie in tuberculosis, and further to the fons et origo of thinking in this context, i.e., to concepts of Metchnikoff (3). The latter's oft quoted phrase regarding a “perfecting of the phagocytic and digestive powers of the leukocytes” might well be, broadly speaking, synonymous with “activation.” Once it was determined that macrophages taken from animals at some appropriate time after infection, for example, with Listerria or Bacillus Calmette-Guérin2 (BCG), were superior to control cells with respect to their antimicrobial powers, it was natural for inquiry to be made into the differences between such cells and normal (control) cells, in terms of their morphology, cellular chemistry, and biochemical attributes.