There are two groups of hypersensitive states (A) all individuals that are adequately exposed become sensitized: including (1) hypersensitiveness of infections, (2) that following parenteral injection of foreign substances, and (3) contact eczema; (B) sensitiveness produced in but a small percentage of the exposed even though exposure be great and repeated. The influence of a tuber-culous lesion in increasing hypersensitiveness to horse serum and egg white suggests a relationship between classes A1 and A2, in that the body is unable to distinguish between the antigens. Anaphylactic hypersensitiveness and atopic hypersensitiveness are similar in the capacity to react with increased intensity to a specific foreign substance; but differ in that (1) anaphylactic reactions can be produced at will, the atopic cannot be thus induced, (2) atopic hypersensitiveness has been observed only in man and anaphylaxis occurs in all animals (3) anaphylactic shock is qualitatively the same in all individuals of a species, in atopic hypersensitiv-ity, localization of symptoms and pathology are variable, (4) atopic is of a much higher degree than anaphylactic hyper-sensitivity. The physiologic mode of action of atopy may be solved by studying the influence of heredity. There is also need to determine (among those who become sensitized) the specific substances to which hypersensitiveness develops.