The extreme chemical heterogeneity of antimicrobial inhibitors may be compared with the large heterogeneity of the sites of attack. From the host of questions regarding the mechanism of action three problems are selected at random for discussion: 1. Penetration or transport of inhibitors into the cell. At least some of the inhibitors must penetrate into the cell and find their way to very specific points of action; and it does not go without saying that the antibiotics reach their targets without previously reacting with other cell components or without being "trapped" by the cytoplasmic membrane. In some instances an active transport, i.e. a "misuse" by antibiotics of a transport system for a substrate can be demonstrated. 2. Distribution of the targets in the cell. Distribution of the inhibitors over the possible targets is not uniform. The question why this is so remains unanswered. 3. The membrane as the point of attack for inhibitors. The probability that an inhibitor has a bactericidal effect also on resting cells is greatest with substances that act on the membrane. The question as to the function of the antibiotics for the active cell arises and the "defence function" of the antibiotics is called in question.